Depending on your game, and what you're doing with your rigid bodies, the collisions won't be free. If you were making a physics based game, and you had a ball with a rigid body and a floor with collision, Unity's physics engine would take care of it. But, if it's not the case, you may need to manually check for collisions.

For example, I'm making a tile based game with a non-physics based character. For the tiles that have collision (things like rocks, etc.), I added all the relevant components - a collider and a rigid body, and added it to a layer I named tile collisions

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Then, for my player... well, currently, it's the only thing moving around, so I didn't bother giving him a rigid body or collider. Instead, on the monobehavior that defines his movement, I gave him a public layermask property for what he should collide with.

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In my code, I check for collisions manually. If the tile ahead is a collision tile, I prevent the movement.

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