How do fold mountains form?
Some mountains form from the collision of tectonic plates; volcanoes are different, and there are some other mechanisms too.

IANAgeologist, and perhaps one will be along to give a more technical explanation, but intuitively, it's about viscosity:

If you push an object very slowly into a bowl of water, nothing much happens - the water flows around the object.

If you push the object equally slowly into a tray of treacle, it's harder work, and you can form a "hill" in front of the object. The treacle flows too slowly to get out of the way.

At very high pressures, rock behaves a bit like the treacle[1] - only much more so. It takes a huge force to push one lump of rock into another, but because the rock flows very slowly, by pushing very slowly you can still create a mountain.

[1] I realise it's not exactly like treacle. If you're an expert here, feel free to post another answer getting it right :-)