When things collide, they transfer their movement energy. If things collide like this >- They will continue in roughly the same direction. If they collide like this -> <- their movement will cancel out and they will fall into the sun.

Satistically, at the “beginning of time”, in a random sphere around the sun, things will not be completely the same. So everything will either collide and fall. Or it will collide and continue in roughly the same direction. What we have now are the leftovers that were moving in roughly the same direction and colliding so little that they didn’t fall into the sun because of that.

The same is true for the “disk”: If you start with a roughly evenly distributed sphere of gases or something, there is a middle somewhere where there is a little bit more mass than anywhere else. That’s where things will go.