Ukraine

Can someone explain how can Russia control that puppet state physically? It neighbours Ukraine, so at least at this point there’s no free passage. Except money, I guess they have some way to pour money onto them, to keep the puppet state alive. But the occupied Ukrainian territories, as much as, say, Belarus, are neighbouring Russia, so it’s somewhat trivial to bring forces, weapons, and other means there. What’s about this region? It’s really difficult to grasp. Perhaps I missed something important, some obvious historical event or something. I did not really read a lot on this, but I’m not sure I know where to begin with. I did read the Wikipedia article (not right now, some time ago), but I’m not sure I extracted much from it. I might give it some refresh, but would really appreciate someone giving it an ELI5-like (well, not really five, obviously) kind of reply.