The only way you can make friends as an adult outside of work would be if you literally put ourself out there. Going to clubs (nightclubs maybe but also hobby type clubs, meetups). Talking to people you don't know in these events. You could also go on dating apps maybe?
Most people keep their school friends or have work friends. Making a real, close friend outside of that is pure luck! A lot of interactions end up as a one time thing or just as exasperated said, acquaintances.
I knew someone who made a close friend off facebook somehow! She was very eager to contact people though. She actually found her husband off a youtube video, somehow got his contact info and now they are married lol. She wasn't a very popular person either, people generally disliked her and she didn't have many friends.