How to make Thonny use installed Python 3.9 (macos)?

Question

I\'m on macos 10.14.6, with Thonny 3.3.3 / Darwin 18.7.0 / Python 3.7.9 / Tk 8.6.8. I\'m using Thonny to program a RaspPi Pico.

Thonny crashes a lot - apparently because