But how do you reliably find large active communities? I’m still new around here! Like if I’m into movie discussions, how do I know if I picked one that has like 5 subs on some obscure small instance vs one with a big subscriber base.
Check out lemmyverse.net/communities, it'll let you search (almost) everything without being limited by your instance, and you can sort by whatever criteria you want.
Here's a site that gives details on instance populations, general rules, functionality, and reliability. https://github.com/maltfield/awesome-lemmy-instances. This site is a couple weeks out of date, but gives a good look into what each instance allows.