I have four separate colliders on the floor (mesh, navmesh obstacle, rigidbody, and terrain) and two on a falling object (rigidbody and sphere collider) and none of them does anything. It falls right through.
Edit: I just went into the model tab and turned on generate colliders for both. Still doesn’t work.
Edit 2: I just noticed an error message saying that non-convex colliders and a non-kinematic rigidbody collider don’t work together anymore. So I turned on kinematic.
Now it doesn’t move at all. I hate this program.
Edit 3: I switched it to convex but not kinematic. It falls straight through again.
Sorry, it’s confusing me a bit… you have a floor that’s made up of mesh + navmesh obstacle + rigidbody + terrain?
And the falling object – is the collider a trigger? Is the rigidbody on the falling object kinematic or non-kinematic?
The first one’s the floor and the second one’s the falling object
I also tried disabling all the colliders except a kinematic rigidbody on the floor and a non-kinematic rigidbody on the falling object, as per this document: Unity - Manual: Introduction to collision
It fell right through yet again and I am now 100% certain that this is not a user error anymore.
I just put in a prefab floor and it works perfectly. As far as I can tell the box collider has the same settings as mine.
I just turned off static on the floor and now it works perfectly. I don’t f*cking understand this program.
Okay, that was confusing. I’m glad you got it sorted out.
You can create a “floor” with just a box collider, no rigidbody required. Just the one collider, too.
A falling object would need a non-kinematic rigidbody + some collider (choose the best primitive that fits the shape).
Anyways, when you’re really new it can be confusing, but try some tutorials and give yourself a little time and patience and you will get a better understanding. 
I was completely lost and confused when I began, too. I don’t think that’s too abnormal.