I've found that if they don't show up in search, you can go directly to the url and subscribe there. After that it will actually show up in search here.
For example, if you find !whatevercommunity@lemmy.ml at browse.feddit.de, but it doesn't appear when you search communities for whatevercommunity, then you can go to the url https://lemmy.perthchat.org/c/whatevercommunity@lemmy.ml and subscribe there.
This has been a bit hit and miss for me though. I'm pretty sure it's supposed to work, but sometimes you'll get a 404. My guess is that it's because the initial request the perthchat instance makes to lemmy.ml might pull down a lot of data, and if lemmy.ml is under load it might drop the request.
Basically, volunteer code commits, volunteer admins, and donations for hosting costs.
Fosstodon is a pretty great example. It's a fairly large mastodon instance which makes enough in donation revenue to pass some on to other open source projects. It's not heaps ($600 in 2021), but I think it demonstrates that donations are a viable funding model. If things got tight I expect the community would meet the challenge.
It's not like you need to build a custom data centre - it's just renting a server, maybe even a VPS.
That said, of course the admins and mods are volunteers. I'd like to imagine that one day a few lemmy instances could charge a subscription fee for a premium, well managed experience.