I generally meet people through other people. I have to get the ball rolling though, so when moving to a new city, I'll generally just throw myself into whatever social situation I can and see what comes of it.
Also, the Internet. I used to meet people of the opposite gender through dating apps and whatnot. Not anymore though.
Also, sporting groups and whatnot. I've done a lot in the way of martial arts over the years and met some pretty interesting people through it.
Also, I often become friends over time with people I see on a regular basis. Examples include baristas, butchers, and the like.
Also, this forum. My IRL friends from here are some of my favourite people.