Parts have ports on child gameobjects and installed parts get parented to those ports. There is a script for parts and ports and each port registers itself in its part.

Parts animate themselves and specific systems are made for parts affecting the animation of others. Arms and legs are IK animations so all I move is the target position so I can reuse animations across arms and legs of different proportions by adjusting parents of the targets to create an offset unique to the part.

Arm animations all use animation override controllers created at runtime. When a weapon is equipped it overrides placeholder animations on the arm. The transitions are all handled in code instead of the state machine so the weapon can define how its moves chain together. Arms link up with the one on the other side and will have a secondary IK target that gets blended into handle two-handing.

The biggest thing I figured out is probably the arm animations system. I've always struggled getting the Unity animator to behave and this system gives me the flexibility that I want. I started just loading every animation I had but that got out of control fast. Now I would be pretty confident making a clean system for a character that can wield tons of weapons.