So the idealistic idea is that there shouldn’t be a “main” community, there should be multiple that are all slightly different in terms of their approach to the topic and you can choose which one(s) best align with what you want.

In reality, at least until the userbase is significantly larger, it actually does make sense to huddle a bit for now and split up topics as needed later. So in this case what I’d do is head to Lemmy Explorer and search for your topic ie https://lemmyverse.net/communities?query=movies

This is quite a broad topic obviously so with a vague keyword search you’re gonna turn up some irrelevant things but there should be some that fit what you need. Then you can change the sort order to whatever you care about most, subscribers, active users, posts etc to decide which is the one you’re most interested in.

Personally I’ve just been subbing to all related communities on a topic, and crossposting my posts to see which one gets the most traction. The little “two squares” icon below the post info is crosspost, works pretty well. Ymmv with that tactic 🙂