In my experience, you have that one friend who uses Twitter a lot, so you look through their mutuals and you discover "oh, these couple other real-life friends

In my experience, you have that one friend who uses Twitter a lot, so you look through their mutuals and you discover "oh, these couple other real-life friends of mine use it," so you follow them any they follow back. You see them retweet or you poke through their followers/following and you see people you don't know, but who post good content, so you follow and maybe interact. Sometimes they follow back. Sometimes you end up DMing them and becoming online or maybe irl friends. Your network continues growing like this until you've got too much on your feed to follow, and you need to take a step back and maybe eliminate some follows. It's a pretty organic thing and you end up with a couple close friends plus a ton of other people semi-connected to your real life friend network.