How to make a normal mesh for a model in unity?

Question

3D

In general, I made a model, saved it in .obj, loaded it into a unit, added a mesh collider, and he made a square collision. How to make the mesh collider be in the form of a model, have a collision and not fall through other objects?? Maybe in a different format you need to save or add a component to the unit? Thanks in advance! On the second screen you can see that there is a white box around the table, this is the collider.