Because when someone is belaying you, and you are going down, how do you get the top carabiner?

usually once you get to the top theres a anchor to put your rope through then on the rappel down you grab your quickdraws (caribiners)

Well, it depends. If you can ''top rope'' it, just walk up there and get them.
If you can not the last person to climb the route, anchors in, goes off belay, strings the rope through the bolt hanger or chain (whatever is on your route), then repels back down. Once down, pull one side of the rope until feeds all the way back down to you.
If you can not get back up to the top.you just lost some carabiners.