How is it that the US can put thousands of objects into LEO, but somehow they have no collision risk?

They actively move to avoid collisions rather than tumbling uncontrollably.

SpaxeX does clean work.

yes they do!

though what has not been brought up is the several ways that everything space x is flying, can be bricked simultaneously, so there failure modes present unique risks

To give an earnest answer, SpaceX deorbits its second stages, and the vast majority of those thousands of satellites are Starlinks which are in a quite low orbit (naturally decaying) and are intentionally deorbited when they reach EOL.

Space is big, man.