When do you use formal vs informal German in everyday life?

My native language doesn’t have a formal/informal distinction like Sie and du, so I’d like to better understand when formal German is actually used in everyday life.

For example, if you’re speaking to a service worker who is clearly younger than you, would you still normally use Sie? Does age matter, or is it more about the situation and the relationship between the people?

I’m curious to hear how you'll decide between Sie and du in everyday situations like shops, restaurants, workplaces, neighbours, strangers, etc.