How does castrating a male affect his singing voice?

It only affects the voice if the male is castrated before puberty.

https://www.classicfm.com/discover-music/what-is-a-castrato/#:~:text=A%20castrato%20is%20a%20male,a%20variety%20of%20other%20effects.

https://en.wikipedia.org/wiki/Castrato

Assuming that castration is done prior to puberty, this will prevent the transformation of the male larynx - normally, on the onset of puberty, the larynx and the vocal chords grow, which leads to the voice becoming lower in tone and deeper. Castration stops this process from occurring, leading to the individual retaining the vocal range of a prepubescent. In addition to the larynx not developing in the normal way, the lack of testosterone also means that the joints do not harden normally, leading to the ribs growing to unusually long lengths. Vocalists subjected to pre-pubescent castration, who are referred to as "castrati", were able to combine these physical characteristics with intensive training to become singers with significant lung capacity (owing to the growth of their ribs) and highly flexible vocal range that was both higher than an ordinary male voice, and different from that of female vocalists.

https://en.wikipedia.org/wiki/Castrato