Someone on my server came up with a mall analogy which I am extending upon, might be helpful:
Sites are like cities (kbin is a city, lemmy is a city, etc)
Instances are like malls (kbin.social is a mall)
Stores inside a mall are like magazines
Cities can have multiple malls, and the malls all talk to each other and give each other information about what's happening in their mall in relation to their stores, which is why we can see posts from other instances of the same site.
And what's more, malls (instances) in different cities (sites) can also talk to malls in another city and pass information about their malls to the other cities' malls. Cities talk to other cities. Translation: The sites share content with each other.

another analogy: Federation in the fediverse is like a group of islands with bridges connecting them. Each island represents a different platform, and the bridges allow people to travel and interact between the islands. Even though each island has its own unique features and rules, the bridges enable communication and sharing of ideas across the entire network of islands.

I hope this didn't further confuse you lol