Question How to keep multiple objects together

Hello everyone,

I really need help with some project I got assigned to by my school. I'm an absolute noob in unity and my programming skills don't go further than some C++ basic knowledge, so sorry in advance if this question is just as noobish.

I've got an digital copy of an industrial machine, which I am supposed to complete for educational purpose in our school. Part of that machine is a workpiece carrier, transported via a conveyor. It consists of several objects like screws, pins, main plate etc and has to transport another part to a gripper.

As soon as I start the simulation, the carrier kind of "explodes", shooting the single parts through the room. I assume the colliders on every single part are too close to each other, giving collision at the very beginning?

How do I tell unity that a bunch of single parts are actually an assembly who have to stick together? I have been messing around with multiple rigidbody, "is kinematic", different colliders, different parenting, I googled as best as I could but I can't put a finger on the solution.
Maybe someone here can end this tragedy for me