Collision in holes( for eg: nut and bolt )

Questions & Answers
1

Hi frriends

I want to inset a cylinder in to a pipe at run time. both the objects should have colliders and one should have rigidbody.

find the attached images of input and desired output for more clarification

2

Just use mesh colliders for both of them. The reason why is because mesh colliders will wrap perfectly around the shape, even if they have holes.

3

you can not create concave meshcolliders in unity anymore.
i suggest you to take a look at this asset.
(please note that i am not owner nor know the owner of this asset)