What’s special about the Castrati?

All children are considered trebles, with the same approximate range as a soprano. It isn’t until puberty that both girls and boys experience a lengthening and thickening of vocal folds that change their vocal range, with males’ folds becoming considerably longer and thicker than females’.
Well, usually, anyway. Castrati were male singers castrated before puberty. Without the normal adult male testosterone levels, they remained natural trebles.
Castrati were often highly paid, and in less enlightened times, some parents castrated their sons in hopes of cashing in.
The only surviving recordings of a castrato performing solo are from 1904 by Alessandro Moreschi. He hits notes common to a soprano with no apparent strain.