To answer your questions about how silencers go onto barrels, yes, they screw onto threads on the end of the barrel.
Some 5.56 suppressors are considered quick detachable which clamp over a flash hider, but you'll still have to thread the barrel so the flash hider will go onto it.
Not many ways around it. I suppose that it's possible to do it in other ways like how a gas block is held in place with taper pins, but I would have to ask: "why?"