Russian here.

I use the formal “You” when talking to adults I don’t know well and in official conversations. Also, with superiors.

I use the informal “you” with friends and family, and with colleagues I know well. Informal “you” also communicates warmth, safety, a call to action, or authority, which is why it may be used when addressing children (particularly preteens), people in danger, or someone else you need to either influence or make feel safe, or both.

Of course there are millions of exceptions, and everyone keeps it slightly different. For this reason, it is common for people to have hard time figuring whether to address certain people by formal or informal “you”. Mistakenly using the formal option can be read as creating more distance, the informal - as invading the personal space. It’s an issue in spoken conversations, too, as these forms are actually two different words that are audibly different.