It's actually unusual for X11 apps to create a window per widget as it comes with a bunch of overhead. Only very old ones do that. However, it does work - windows can be used as widgets and they can cross-embed from other processes. This is very cool but rarely used. It allows you to run a casually sandboxed subprocess with GUI I/O.

They can also just draw on each other's windows and get input events on each other's windows. The connection between a window and its creator process is very weak: the window is automatically destroyed when the creator disconnects. And that's all. You can even turn that off, too. For every request that includes a window (or other resource) ID, nothing says that it has to be your window.