Suppose you push on a heavy crate, but not hard enough to make it slide. Does a friction force act on the crate? a. No, the friction force doesn't act.
b. Yes, opposite to your push, just enough so that F = 0.

The net force is 0 because the friction force counteracts or counterbalances the force you apply, which is why the object does not move. Specifically the static friction is holding the object in place and keeping the object static.

Once you apply enough force and overcome that static friction, then the object will move and you'll be dealing with dynamic friction. Dynamic friction is what still holds the object back somewhat and allows the object to slow down and eventually stop (when the force you apply isn't enough anymore).

Without dynamic friction, the object would slide forever until it hit the wall. With something like slippery ice, there is dynamic friction but it's not as large compared to other surfaces.

no

sodium is solid at rt