The core loop of new friend formation is repeated casual contact.

School is naturally good at forcing this, since we are filtering in and out of different class groups.

If you want to recreate this in post-school life, you have to join high membership interest groups, like casual sports groups (kickball, tennis, golf).

Game nights at the local pub work well too, since there will be a rotating cast of characters to make friends with.

Basically, you have to seek novelty and regular re-occurance to make "natural" new friends.

Yeah I get it, I'm into cars (drive fast, don't change my own oil) and started talking to a guy at work about his car, had me sit in it (Porsche). Pretty sick. It definitely helps to have a shared interest.