I know what industry they work at, but if they switched to a different employer, I might only find out months or years later sometimes because we almost never talk about work outside of work unless someone asks for advice.
Birthdays are actually quite easy, every time I learn of someone’s birthday, I put it on an app called Birday and I get a notification two weeks before their birthday. Before that, I missed people’s birthdays regularly.
I know if they are single/have a partner/married, have kids or not, or are divorced, but I might not know if they have had an argument with their partner and are sleeping on the couch or something.
At the same time, I have a lot of personal things I don’t share with anyone, and I don’t expect them to share it with me. I just prefer it this way.