UPDATE: Unfortunately, I haven't been able to make the Arduino board show up in Windows as well (tried Windows 10).
I've ordered a new board which arrived today!
I've tried it now and it worked right out of the box on my Macbook running Catalina v10.15.5, even after plugging it in and out several times and uploading different sketches.
It shows up in my system as /dev/cu.usbmodem14201.
After this I've tried plugging in the 'old' board again, but it still isn't recognized by my system.
I'll keep the 'old' board around to try some more things in the future.
One interesting difference that I've noticed in the two boards : when I plug in the 'old' one to USB power, the mult-colored LED lights up.
This doesn't happen with the 'new' one, i.e. the multi-colored LED stays black.
Is this a hint to what might be wrong with the 'old' board?
... or is this a sign that the 'new' board is also defective 
?