are the parts to build companion or to help build me i dont know just started game first time

If you're referring to the corpse arm you can get off a particularly fragile zombie, it's nothing but an improvised weapon (and not a very good one, even).

[SPOILER]

There is a particular severed limb you can discover later in the game that does have some significance, but learning about it requires showing it to a certain NPC with a certain party member present during the conversation (along with high mental stats to figure out what's going on). The limb itself is still just a club, though. There are also quests where the best outcome requires abusing your regeneration ability (notice how one of your equipment slots is your eye).

Fun fact: originally, you were supposed to be able to tear off your own arm and use it as a weapon. This was never implemented, but there's a reference to this mechanic near the end of the game.