your description of the issue is very unusual. Before getting to the Isadora issues, you'd need to solve the resolution in MacOS (Big Sur).
If you get a picture of the desktop on the external projector, there has to be a possibility to configure it in the 'Display' menu.
I don't have the Big Sur version here to check it. But as far as I remember, the system preferences menu is called 'Displays' not screens.
If the projector is recognized by the OS, AND mirroring is deactivated, there should open one window on every screen / projection, connected. If you can't see those windows, there is a button on the main screen window, saying 'Gather windows'.
Path to the display settings:
Apple symbol on top left -> system preferences -> Displays
In the appearing separately extra window, which is dedicated to the external device (projector) you need to change to 'scaled'. A List of possible resolutions should appear.