qimgv uses libmpv for video playback support for example. I'm guessing that's not what you mean, but I'm struggling to think of how one might "embed" one application inside of another on xorg.

x11 supports foreign window embedding. You can embed window from other application into your own window. That's why lots of mpv/vlc based players/editors doesn't work probably on wayland. The only way to achieve this on wayland is writing a custom compositor for the foreign window.