!newcommunities@lemmy.world is a very common one. I'll put a plug also for !communities@ponder.cat. I also tend to look at the lists of communities from "good" instances and subscribe en masse to a bunch of them all at once if I like the vibe of the instance overall.

You are right that there really isn't as good a way as there "should" be, maybe, but that's what people do and it seems to work okay.

https://lemmyverse.net/communities

Or copying subscriptions from another (old) account someplace else.

If you are talking about some brand new instance that you just created, you can construct a URL with the community name to find it the first time, then someone from your instance needs to be the first to subscribe/join it. After that, someone just needs to search for it using its name from the Communities link at the top of your instance homepage - making sure that All rather than Local is selected at the top.

PieFed will pull in old posts, I think Lemmy won't, but either way the old comments and votes are a lost cause, however newer ones (after possibly a bit of a delay, maybe 24 hours unless there's a larger backlog of things to sync) will automatically sync up from then on (additional caveats exist like locked/removed posts).

So for example to get to the !AskUSA@discuss.online community, you'd go to https://yourinstanceurlhere/c/AskUSA@discuss.online and join (e.g. https://lemm.ee/c/AskUSA@discuss.online is a functional link).

Caveat: the instance (Discuss.Online in the aforementioned example) must already be federated with, which you should see in the https://yourinstanceurlhere/instances, and not also defederated with (lemm.ee barely defederates from anything, but most instances have a much more substantial list of spammers and gore and bad-faith instances that they curate so their users don't have to do so individually). An easy way to get there: go to homepage, scroll all the way down and click Instances; to see the defederated list click the Blocked Instances tab at the top of that.

But from your current instance lemm.ee, all of this has almost certainly been done for you, as iirc it is the #3 largest instance across the entire Fediverse. Just make sure that searching the Communities list is set to All rather than Local!:-)

Edit: but as for how to discover the community names in the first place, simply browse some existing instance like lemm.ee, you don't even need an account to see most of them (those not marked as internal-only).

besides the communities or websites to find communities, as other have already posted. Some instances and communities add their community to https://lemmy-federate.com/. A project using a bot that initaly joins a community from different instances, so you are able so find them easier.

Just surf along the information superhighway and keep an eye out for tasty links. You'll soon get the hang of it. Here's one place to start: https://mastodon.social/public/remote

many people just browse the /all/newest feed which is an aggregate of all subscriptions on the instance, and then you can subscribe from those posts

peruse all and block communities you have zero interest in while subscribing to things you like. Eventually to check your subscriptions first and when caught up you check all to see if anything new catches your eye.

Global feed. 'All' on Lemmy, 'Federated timeline' on Mastodon, etc., for established instances.

Trolling big instances and copying account URLs on clean instances.

Edit: Wanna tell me what's wrong with that? Or are we just passive-aggressive and not worth talking to?