non-parent-child constraint

hello together,

I am building/simulating a robot arm, which needs a kind of constraint:
constraint includes two parts for example A and B. A and B can influence position and rotation (euler angle) of each other , which means the constraint has no parent-child relationship of the parts.(but can have temp SetParent in Update or FixedUpdate)
It can be fixed / revolute / translational constraint.

Attention please, the base should be fixed to the ground(global), which means: during the ik process, there can be conflict between each part: the base (fixed) tries determining the end and vice versa...

I have tried the unity built in Joints and it works well, but I'd like to build relative simplified classes without physic model, just kinematics relationship.

I have searched almost all the possible github open repos and there is no solution.

Could you guys please give me some suggestions, thx a lot.