because all the instances are federated, they don’t download information from another instance until it is necessary (to save on bandwidth and server space)
instances become linked when a user from one instance subscribes to a community/magazine on another instance or when they try searching for content on another instance
this is also why there’s a delay when searching “All”, gotta give the instance a little bit to go out and start grabbing content – and this is why searches come back with nothing but if you just wait, suddenly it starts populating half a minute later
you can see which instances your instance is already linked to (and which instances your instance is blocking) by adding /instances to the end of your instance’s url – ex. lemmy.world/instances