Hey, hey. I spent some more time reading about this and I had it wrong - I thought Lemmy federated users, but instead it does federate some data across instances. As ronnypopiel stated, text content will federate across instances while other media is shared as links to the original instance.
Getting Started with Federation; Federation for Contributors; Another discussion on this topic on kBin
Does this mean that when you create an instance, you have to retroactively download all of the data? Or is it simply copied to all the conneced instances as it's made?
Not quite. The data isn't directly copie. Imagine that a shortcut is created that links to the original instance the data was posted to in the first place.
Federation means that your account on one instance allows you to connect to and browse content on other instances, without signing into the other instances. The instances are "federated" with each other, which means that they trust the accounts on those other instances.
The latches are sometimes stepped and might not hold the valve open all the way, so it's possible it's slower compared to holding it all the way open.
Great stuff. Thanks!