Depending on the phone, that can be relatively easy.
What I mean is putting modem and oem files from Xperia 10 onto XA2 (Ultra) to attempt to get working T-Mobile VOLTE. Newflasher won’t do it, so trying manually doing it, but I seemingly can’t write to /oem, and recovery doesn’t pick it up.
I meant relatively as it’s more of a nerd thing to use custom ROMs, and you would usually have more experience with this kind of thing.
My issues are things nobody has cared enough to try, or impossible to fix idk.