When I visit kbin.social, I see new posts regularly. On other servers posts stay on the frontpage for multiple days. This is also true if I switch their sorting to “hot”. So that is probably not the difference.
What is kbin.social doing differently?
Thanks guys for all of your answers! I understand now the ActivityPub is the protocol that facilitates the communication between instances. Also I get it now that lemmy just offers the software part if you want to spin up your own instance without writing all the front end and back end from scratch.