Most parts are sold incomplete. Say, a leg also needs a foot, and an arm also needs a hand. There's a button in the prosthesis's own interface as an item, that offers you to pick a desired version of said prosthesis. If you pick complete, you'll need to source other parts as well and combine them (a new button will appear there). You can also install an incomplete version, there's a button on the surgery that offers to commence an installation of an incomplete part, but it won't install the missing links, as you might imagine.

So, an arm requires to combine:
1. Arm
1.1 Internal support
1.2 Forearm
1.2.1 Internal support x2
1.2.2 Hand
1.2.3.1 Five fingers

Total: 11 parts

@noj , could you please add something similar to the mod's description. Understanding this takes quite some time from within the game. It would also be quite awesome to have a list of sub parts for every appendage.