In RE7 is explained throughout the various notes that the Healing med is powerful enough to put limbs back together. Also later it in RE8 explains why Ethan can take so much stabbing and not die, which in turns helps with the whole "Immah stick my hand back into my arm" deal.

It's also important to notice that since Resident Evil 2 it's canon that the advancement of biological research was pushed so far by Umbrella that medicine is able to cure issues like baldness and limb loss on the fly