Simulator is a black screen
So I made it work in the end, yet I am not very clear with the root cause of the problem.  Any further information is more than welcome.
I found that the problem (Blank screen) only happened when I wanted to emulate an iPhone 11, so I thought it was related to that one simulator.  Using the simctl I deleted all the unavailable emulators:
xcrun simctl delete unavailable
Then in Xcode, in a new project lean enough to respond &quot;in time&quot;, I went to the device manager and deleted my iPhone 11 emulator, then created a new one of the same model with the same iOS version.
After that, the Main.storyboard began to display properly for any model I could choose and the emulators were able to boot any of my applications.
I don't know why this happen, I don't know how to reproduce this, but I know that now it works.  I hope this helps someone in the future.