yeah most of my followers come from the fanbases im a part of, and they can find you through mutual friends or by participating in trends. or when celebs follow/tweet you, some of their fans will see it and follow you (i gained literally 100 or so followers when Austin followed me and lots when Ariana tweeted)
but yeah sometimes super random people follow me who i have nothing in common with and i have no idea how they even found me

i keep my following list to around 300 because i like to actually read my timeline, and when you follow more than that it gets messy. i kinda feel bad for not following everyone back but oh well. if you ask me for a follow back i usually will