Long winded explanation but hopefully it clears a few things up...

There is a protocol that allows two or more things to talk to each other. That is called ActivityPub and things that speak that protocol can work together. The things in this case are the servers which you register your account with.

https://en.wikipedia.org/wiki/ActivityPub

You can have two different things (kbin and a Lemmy instance) and they can work together despite being different. They are differently written programs in different programming languages. The ActivityPub federation thing allows your Lemmy instance to ask to be sent all posts and comments from another server "just to have" as a local copy which you can then read on your server. If you comment, vote, or post on your server's "local copy" then your server sends that update back to the original server so they can update their records.

The content being downloaded from a remote server is like 1 person looking at everything over there. At that point, you and everyone else on your server can look at your local copy which is quick (if your server is not overloaded). The remote server can better handle lots of users from where you are at because your server is taking on the "user interaction load". This way, the user load is distributed to remote servers where the users do their interactions and the smaller "like 1 person" transactions are sent along when necessary.

The way things work on the Lemmy side is that you can see other communities by using the search function or by clicking on the communities button and clicking on all. You will see all communities that your server "knows about" (including communities hosted on this specific server). It automatically downloads all content from remote servers that it "knows about" (and it does so frequently) but that is driven by you first asking the server to get things from a specific server if necessary.

You can ask for things by searching for a specific address starting with a ! character. Wait a few moments and search again and you should be able to see that the content was retrieved for you. (You can search by a more granular term like the server address itself if necessary.) At that point, you can interact with that community in expected ways (like subscribing to that community to see updates from them).

The link you need is on the right side of any community page you view. Examples are technology@beehaw.org and kbinMeta@kbin.social

Once everything is set up and working, the server you log in to will automatically gather posts and comments from "the fediverse", show it on your screen, and send along any comments or votes you make back to "the fediverse" for others to see.

That all being said... kbin is a bit overloaded at the moment and is not quite sending updates to other servers. When they have that sorted, you should be able to interact with them. Refer to the following post from chaorace for more information.

https://lemmy.sdf.org/comment/67263