Discord for Android has a useful feature in which a button for its voice-chat functions can float over every other aspect of the UI. Unfortunately, it was originally launched as an exclusive to the Samsung Galaxy Notes 10. However, it has now been detected as working in more Android phones via the app's latest betas.
https://www.notebookcheck.net/Discord-is-rolling-its-Mobile-Voice-Overlay-feature-out-to-non-Samsung-phones.463025.0.html