How does a bike stop?

Don't really know what you're after here...

The bicycle as a whole keeps moving due to something called inertia. Inertia means that an item prefers to keep doing what it was doing the moment before. If it was stationary it takes an effort to get it moving, if it's moving it takes an effort to stop it.

Unless you're heading downhill, or have a strong tailwind, in which case it's gravity or the tailwind that keeps you moving.

THe other posssible thing you're after is the one-way clutch thingy stuck in most rear wheels. Depending on bike it'scalled either a freewheel or a freehub, and it allows the pedals to hook on when pushed forward, and to disengage if the wheel is moving by itself.