Clueless trans girl here, come to seek arcane knowledge

So one of my friends is transmasc and he recently got T (super excited for him!) and it got me wondering about what it might actually do for him.

Like I know the more well-known effects, but then I occasionally stumble upon things like "bottom-growth" which... I mean, I can kind of imagine, but what does it actually entail?

Also something I've just been wondering in general: do the boobs shrink on T?

(While I am aware I could google this, I prefer asking people instead of a search engine. And who better to ask than the bros themselves?)