Lemmy uses the ActivityPub protocol (a W3C standard) to enable federation between different servers (often called instances). This is very similar to the way email works. For example, if you use gmail.com, then you can not only send mails to other gmail.com users, but also to yahoo.com, yandex.ru and so on. Email uses the SMTP protocol to achieve this, so you can think of ActivityPub as "SMTP for social media". The amount of different actions possible on social media (post, comment, like, share, etc) means that ActivityPub is much more complicated than SMTP.
As with email, ActivityPub federation happens only between servers. So if you are registered on
enterprise.lemmy.ml, you only connect to the API of
enterprise.lemmy.ml, while the server takes care of sending and receiving data from other instances (eg
voyager.lemmy.ml). The great advantage of this approach is that the average user doesn't have to do anything to use federation. In fact if you are using Lemmy, you are likely already using it. One way to confirm is by going to a community or user profile. If you are on
enterprise.lemmy.ml and you see a user like
@email@example.com, or a community like
!firstname.lastname@example.org, then those are federated, meaning they use a different instance from yours.
One way you can take advantage of federation is by opening a different instance, like
ds9.lemmy.ml, and browsing it. If you see an interesting community, post or user that you want to interact with, just copy its URL and paste it into the search of your own instance. Your instance will connect to the other one (assuming the allowlist/blocklist allows it), and directly display the remote content to you, so that you can follow a community or comment on a post. Here are some examples of working searches:
You can see the list of linked instances by following the "Instances" link at the bottom of any Lemmy page.
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
- Post, comment edits and deletions
- Mod actions
You can copy the URL of the community from the address bar in your browser and insert it in your search field. Wait a few seconds, the post will appear below. At the moment there is no loading indicator for the search, so wait a few seconds if it shows "no results".
Paste the URL of a post into your Lemmy instance's search field. Wait a few seconds until the post appears. This will also fetch the community profile, and the profile of the post creator.
If you find an interesting comment under a posting on another instance, you can find below the comment in the 3-dot-menu the link-symbol. Copy this link. It looks like
https://lemmy.ml/post/56382/comment/40796. Remove the
post/XXX part and put it into your search-bar. For this example, search for
https://lemmy.ml/comment/40796. This comment, all parent comments, users and community and the corresponding post are fetched from the remote instance, if they are not known locally.
Sibling comments are not fetched! If you want more comments from older posts, you have to search for each of them as described above.