I'm able to reproduce this issue every time I select the expand window button in the RingCentral app running on Big Sur. Once I do, it almost looks like it enables 'Spaces' and completely takes over the entire screen, locking me out from launching any apps. I can get back to using any of my other desktop apps until I force quit RingCentral.
I have Mission Control and Spaces all turned off. The only work around has been to delete the app including the Application support files and reinstalling. But once I click on the expand screen button after reinstalling, the problem comes back again, locking me out from running any other apps until I force quit RingCentral.