I could answer my own question, actually!

For reference, I’m in western austria, speaking German. The class I’m taking is A2 French.

My region is pretty different from most of the German speaking ‘world’. We use the formal you much less. The informal one is more or less th default, except:

You’re in secondary school. The teachers will use the informal one for students and the students have to use the formal one for most teachers. In high school, students can technically request that teachers use the formal you for them, but nobody does. I teach night school, and nobody used the formal you. Most of my students are very roughly around my age.

You’re seeing a doctor you don’t repeatedly go to, e.g. at the hospital. We use informal you for the specialists and GPs we see regularly, unless they’re ~60+.

You’re a bachelor’s student. Formal you for both students and professors. Unless the teacher is a masters or PhD student, then informal you both ways. Masters and PhD students tend to use informal you with professors and vice versa, but some professors will be the exception and there will be formal you both ways.

Court. Formal you, except between a lawyer and their client.

Some stuffy, old fashioned workplaces use formal you, but only between boss and employees, very very rarely between employees. If it’s some higher level management person you don’t usually work with, it’s more likely you’ll use formal you both ways.

Super specific, but 80+ year old people who’ve never lived outside a city will want kids to use formal you for them, but they’ll use the informal one for the kids.

German tourists. We’re aware that informal you is more common in Germany, and try to me courteous. Except those of us who hate tourists, lol.

That’s all the exceptions I can think of! For everyone else, including strangers (e.g. when asking for directions, cashiers, waiters, etc.) we use the informal one!