How can I select from an open dialog an ".app" that is part of the contents of another ".app"?
In addition to the answer @tetsujin gave you in the comment, you can press cmd+shift+G in the open dialog to open an interface that lets you navigate to arbitrary folders. This has just been updated in Monterey (works with previous versions, it's just cooler in Monterey as it has a very helpful autocompletion).
You can use this trick in the Finder too, it's very useful to go look for stuff inside your Library folder.