Childhood, college, work, going to see small local bands, DJing, hanging around bars or being a local in a little pub, just talking to people.

Lots of board game shops have events where people just come to play board games. It’s often easier meeting strangers if you’re all there already doing some shared interest. Likewise with outdoor pursuits etc.

A thing I don’t see mentioned very often is that most of my lasting friendships aren’t with the first person in a group I got to know, but one of their friends that I actually had more in common with. There’s plenty of casual friends or just acquaintances I hung out with a few times despite not having too much in common or clicking right away, only to then be introduced at some event or one night in the pub to their other friends and finding I really hit it off with one of them.