"If you search for a community first time, 20 posts are fetched initially. Only if a least one user on your instance subscribes to the remote community, will the community send updates to your instance. Updates include:

New posts, comments
Votes
Post, comment edits and deletions
Mod actions"

So you create a single user and subscribe to all communities of interest.

I probably downplayed the difficulty of setting up a Lemmy instance that will come if you do something out of order or don't quite have the host set up correctly or something. Although I do think it's easier than pigging about with web crawlers.