How PieFed federates feeds (aka multi-reddits or multi-comms)
Recently PieFed added a way to group communities into collections of related communities, which we called a “Feed”. Unlike Topics, Feeds are federated, can be created by anyone and can be public or private. There are now hundreds of feeds at https://piefed.social/feeds.
A feed being federated means that people using other instances can subscribe to feeds that were created on your instance, in the same way that people can subscribe/join communities on remote instances.
I’ve written up the technical details of how this all works behind the scenes at:
https://codeberg.org/rimu/pyfedi/src/branch/main/docs/activitypub_examples/feeds.md