Looking Through a "Glass Onion" Quiz

Answer: the Covid-19 pandemic

There are a few references to the pandemic at the beginning of the movie including Claire (who is Governor of Connecticut) holding a press interview from her home (work from home), Benoit talking about how crazy he is going without a case to work on and Birdie being rebuked for having a party. She replies that it's alright because all these people are "in her pod".
When the guests arrive on the dock, they are wearing masks, although Birdie's is made of open mesh. A man arrives and give each of them a shot of something in their mouths, telling them, immediately, that they are alright, thus disposing of the need to constantly take precautions. They then board the boat and head for the island.
Miles Bron is played by Edward Norton, Benoit Blanc by Daniel Craig, Claire Debella by Kathryn Hahn, Lionel Toussaint is played by Leslie Odom Jr, Andi Brand is played by Janelle Monae, and Birdie Jay by Kate Hudson. Duke Cody is played by Dave Bautista, Peg by Jessica Henwick and Whiskey is played by Madelyn Cline.

Answer: in the bathtub

Blanc is in the bathtub, smoking a cigar and playing on online game with four friends. He loses the game causing a friend to comment that he expected the 'world's greatest detective' to do better. The online friends express concern about how he's handling the pandemic, saying that his friend Phillip claims that he hasn't left the tub in weeks, which Benoit says is hyperbole.
There is a knock on the door and Benoit asks Phillip to answer, who replies with "You're not in the bath again are you?"
The news that there is someone to see him who has brought him a box gets him interested enough to get out of the tub and get dressed in his dressing gown (with a shirt and tie underneath). Phillip is played by Hugh Grant.
This scene includes a few cameo appearances by celebrities with a few more scattered throughout the movie.

Answer: Yes

The consummate host, Miles only looks baffled for a minute at Benoit's appearance on the island, but after everyone disperses to their rooms, he asks Benoit if they can have a word. Once they get to his office, Miles asks Benoit what he is doing there.

A conversation follows in which Benoit claims that Miles invited him, which Miles denies. Talk of the invitation box and Benoit's production of the printed invitation found inside puzzles Miles, until Benoit raises the possibility that the box was reset. Miles accepts the fact that one of his friends did this and then sent it to Benoit "as a gag". Benoit claims to be mortified by not having been invited, so Miles officially invites Benoit, telling him to try and solve the mystery.

Answer: Because there are no roads on the island

When Benoit and Miles are in Mile's office, discussing Benoit's presence on the island, Benoit notices Miles' car, a blue Porsche 918 Spyder, and asks about it. When questioned about the car, Miles' identifies it as his "baby blue" saying he never goes anywhere without it. Benoit then asks why it is on the roof and Miles says that there are no roads on the island, as if Benoit should have figured this out.
The car later becomes significant in the unravelling of the real murder.

Answer: Cassandra (Andi) Brand

Andi was one of the original 'disruptors' and a partner to Miles when he built his fortune with the tech company, Alpha. In fact she was the one who had the original concept for the business which she had written on a napkin from their favourite bar. It was a huge success. When Miles became obsessed with his new, hydrogen energy source, Klear, Andi refused to get behind it because it was too dangerous. Miles had his lawyers close Andi out of the partnership, claiming that the idea for the company was his alone. Each one of the group testified, in court, that it was Miles' idea, leaving Andi out in the cold. Because of this history, they are surprised and uncomfortable to see her arrive on the island.

This is shown on their faces and with comments made throughout the movie.

Answer: Birdie Jay

Once Miles announces that it is time to turn their attentions to the purpose of the weekend, the murder mystery, Benoit asks if they have started. Miles replies that the murder hasn't actually happened yet but tells him to go ahead and give it a shot.
Benoit identifies the murderer as Birdie who, he surmises, planted a remote on a crossbow that is pointed at Miles, in revenge for Miles having stolen her signature Ren diamond. He identifies the murder weapon as a J. Hawke crossbow, which is a play on Birdie Jay. He talks about other clues, including an old magazine left lying around, where Birdie can be seen on the cover wearing the diamond. The piece de resistance is when Benoit identifies the locket that Miles wears around his neck as containing the diamond. Miles is unimpressed.
The crossbow, firing and hitting Miles in the chest where he is obviously wearing a bag of fake blood, supports Benoit's solution.

Answer: pineapple in his drink

When they first arrive on the island and are presented with the shot in the mouth that makes them immediately safe from Covid, Duke hesitates before opening in his mouth, making sure that it didn't contain pineapple because, "Duke don't dance with pineapple."

Just before the murder happens, Duke gets notices on his cell phone that the real Andi Brand is dead, something which had not been made public. Duke remembers almost being 'pancaked' on his motorcycle, on the night of Andi's death, by Miles in his recognisable 'baby blue' Porsche. Duke was on his way to talk to Andi and Miles was on his way back from having killed her, because she had located the original napkin that the idea for Alpha was written on. Until Duke receives the notices from social media, he doesn't know that Andi is dead so doesn't realise the significance of Miles being there that night.
Earlier in the movie he asked Miles to put him on "Alpha News" but Miles said Duke's "numbers" weren't high enough. Duke claims that his "numbers" have exploded and shows Miles his phone with a notice about Andi's death. Duke then says that this changes everything and he should be able to go on "Alpha News"
and Miles agrees, embracing him and patting him on the back before adding pineapple juice to his own drink. Then, getting Birdie to distract with dancing, Miles hands Duke his pineapple-laden drink and takes Duke's drink. After Duke is dead, Miles notices that he was using Mile's glass and claims that the poison was meant for him, offering Benoit money to find out who tried to kill him.

Answer: Helen Brand

Helen is Andi's twin sister and, because Andi is dead, she receives the puzzle box sent to Andi at the beginning of the movie. She doesn't waste time solving the puzzles, but opens it with a hammer, retrieving the invitation card inside. She then takes both the remains of the box and the invitation to Benoit Blanc (freshly out of the bathtub). She hires him to find out who killed her sister, explaining the history of the group and the significance of the napkin, which her sister had found, announcing this find to the group in emails.
Benoit is very clear that while he could figure out who killed Andi, his jurisdiction ended there and he couldn't bring the person to justice. He encouraged Helen to pretend to be her sister, explaining that her presence would upset them, especially whoever thought he or she had killed her.
Benoit later regrets this decision when Helen, still appearing as her sister, is shot through a window. However it turns out that Helen is not dead, because the bullet lodged in her sister's diary, which was in her pocket. Benoit encourages her to 'play dead' as this would make her free to roam around looking for the napkin, which they know from a picture, Andi had put in a red envelope. So they sprinkle hot sauce on her and mourn her death with the rest of the group, giving her free rein to explore when left alone.

Answer: a vainglorious buffoon

Benoit speaks about the thing that has bothered him since the beginning of the case, specifically that although everyone (including himself initially) assumes that Miles is a "complicated genius", he is, in fact, an idiot. He talks about all the words and expressions Miles uses that aren't real, like "inbreviate", "infraction point" and "predefinite detective"' and words that are real words but misused and complete misinformation. He points out Miles is "a minefield of misinformation and malapropisms" and even mentions that Miles neither designed the puzzle boxes nor wrote the murder mystery, having hired people to do so.

Obviously unhappy, Miles then asks Benoit to please identify who tried to kill him and Benoit says that nobody did, calling him a "vainglorious buffoon", before laying the explanation of the murders out.

Answer: He burns it.

If not the genius he thinks he is, Miles is definitely arrogant. Instead of destroying the napkin when he killed Andi, he took it and hid it behind a picture of the gang. Helen finds it, when everyone assumes that she is dead. Miles claims that it is the fake but she points out that the napkin with Andi's writing had the stamp of the bar they frequented (Glass Onion) which has been closed for 9 years. As he steps close enough for him to see this, he whips out a lighter and burns it. Then he expects all of his friends to lie for him as they had done on the stand and initially they don't stand up to him.
Helen then starts smashing much of the glass in the room while he looks on in amusement. Soon some of the "disruptors" get involved in the smashing, including Miles himself. Then Helen throws a piece of Klear, which has been given to her by Benoit (his new energy source), into a fire she had started and things start to blow up. This proves that it is unstable and unsafe, as he is using it to power the whole island.
While everything is burning, Helen makes a move for the switch that cancels the protection of the Mona Lisa, which he has "rented" from the Louvre. He tries to stop her but she gets there first and the painting goes up in flames.
Ironically this grants his earlier stated desire to have his name said in the same breath as the name of the painting.