Earlier quoted context omitted.
I always struggled with this in Germany as a foreigner. Coming from a country in which the so called formal form changed significantly in the last decades and is now reserved to people you don't want to get along with (that's how politicians refer to members of other parties, or how customer support starts to address you after you tell them that you want to quit the contract), I always ended up using the informal for…
Many people who formally learn German as a second language (as opposed to just picking it up on the spot) seem to default to Sie. I guess that's what they teach in the Goethe institute. Defaulting to Sie seems like a safe default. At worst you get some funny looks for being overly formal, but you don't insult anyone.
It’s not just knowing when to use formal/informal - native speakers often forget this but in languages with verbs that conjugate, a beginner learner may not be as good with the Du conjugations as they are with Sie. Again, when you are just starting with the language or you’re not confident you lean on repeating things you’ve learned and if you were taught Sie then that’s what you do.