MTF: How does fat repositioning(?) work?

(?) because I’m not sure if that’s quite the right term I’m curious how the actual changes to your body work. Do you have to lose/gain weight for most of them (I know you had to gain weight for breasts?) or will your figure change if you’re eating as much as normal? In either case, it’d probably be best to be slimmer by the time you start HRT right? But worse to put off HRT to get slimmer?