Why does a full screen application black out the other display?

When I go into (at least some) full screen app(s) the other display gets blacked out. An example is Zoom. Is there a workaround for this that would allow me to continue using the unused display for other applications?

Note there is a similar question here : Is there a way to run one application in Full Screen mode on one display and still be able to view and access applications on a second display? . However that question is very old (2012 OS/X Lion) and does not have any OS/X builtin answer but instead depends on a third party library.

I am on catalina on a macbook PRO.

Solution 1:

Enable the setting System Preferences > Mission Control > Displays have separate Spaces:

It requires you to log out/back in, and then you will be able to use other displays as regular when an app is full screen in one of the displays.