Maybe I can be of help with explaining how Lemmy and the federation works. (Hope I have this right, anybody feel free to correct me)
Lemmy itself is just the software used by the different servers (beehaw.org, sh.itjust.works, etc..) that belong to the federation.
These servers can each have communities and users that belong to them, but these communities and users can all interact with and be interacted with from other servers. Example, I am currently logged into and browsing this post from the sh.itjust.works server)
So the communities that you see on lemmy.one are hosted on one server and the communities from beehaw.org are hosted on a different server.
Because of this setup, communities aren't just as simple as Reddit (/r/wellthatsucks) because there can be !wellthatsucks and !wellthatsucks. These communities are different and run by different people.
Now the way the federation for the servers works is that in order for servers to know that eachother exist, communities or posts from one server have to be searched for on another one. (Ex. searching !wellthatsucks while on beehaw.org) by searching for the communities, your server now knows they exist and will work in the background to sync those communities so that you can browse them from your own server.
The idea is that you only need 1 account on 1 server somewhere in order to participate in the federation.
Unfortunately, because of this system and the rapid growth from the Reddit exodus, communities have been splintered onto different servers. That is why LOTRmemes exist in many different places. They are all different communities.
In order to find the communities that you are looking for on beehaw, search for them while logged into your account on Lemmy.one. You'll probably be able to find them.
Keep in mind, this federation works when the servers are okay with communicating with eachother. You will find there are posts talking about defederation of servers.
My home server is has been defederated from beehaw.org. I cannot find their communities or interact with anything on their server, and the same with them to my home server. Both of our servers however can still participate with other servers though, because only the metaphorical link between beehaw.org and sh.itjust.works is broken, all the others are still intact.
I hope this sorta clears things up a bit for you. Welcome to Lemmy!