When you set up a lemmy instance, it has no idea other instances exist. It's like throwing you into a web browser with no search engine. You don't immediately see every single website, you have someone tell you about a cool website you found, and then you type it into the address bar, and save it.
It's kind of the same thing with Lemmy instances and communities. Once a user types this syntax into the search prompt:
!community@instance.com
It will try and contact instance.com for that community. If it exists, the user can subscribe and the instances will now receive and send new posts to each other.
Since no one answered: I think the users on the instance (copy of Lemmy) need to federate (connect) with other instances in order for the first instance to be federated (semi-permanently in communication) with those other instances.
I am sure more knowledgeable people will provide the right answers.
But I think it means that when you set up your own lemmy instance, you need to add the communities to it.
when you set up an instance, all will be empty. the moment an user subscribe to a community for the first time, that community will start appearing on all, for everyone to see.
for this reason all is different for every instance.
the motivation for it is resources. if a new instance would receive updates from all the communities on all the instances, it would be very much like ddos. and a small instance will not be able to endure it.