How did St. Nicholas get the name Santa Claus?

Saint Nicholas' shortened Dutch nickname was "Sinter Klaas." From there, it's a short stretch to "Santa Claus."

https://www.sttammanylibrary.org/blogs/post/the-legend-of-santa-claus-part-1-the-origin-and-history-of-a-christmas-legend/

Dutch families brought the tradition of celebrating St. Nicholas to New Amsterdam in the American colonies as early as the 17th century. They called him Sinterklaas. This name became Santa Claus for the English-speaking majority of the early United States. The legend of a kindly old man was combined with old Nordic folk tales of a magician who punished naughty children and rewarded good ones with gifts to create the pop cultural figure of Santa Claus.

https://www.britannica.com/question/How-did-Saint-Nicholas-inspire-the-creation-of-Santa-Claus