I found another, quicker way to recover the apps on my main screen.
In the system settings, change the main monitor to the other screen back to the original screen. Suddenly, the apps appear again.
romankk
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Same here after updating to Big Sur. As soon as I connect the external monitor, all my apps are somewhere off-screen. I have to use Mission Control to drag my apps to the laptop screen to access them again. This is quite a nuisance.