MacBook wasn’t booting anymore

About a year ago i installed Sonoma on this MacBook Air and it worked, slowly, but fine. Today i wanted to get JIT working on my phone so i turned it back on, and it always booted into Windows. So what i think happened is that the MacBook checked that the version isn’t supported on it. I got an Forbidden 🚫 icon.

Im on the way installing Big Sur again but i would like to hear what happend. I didn’t change anything nor used it when i installed everything.