Think about it

Seeds are basically different universes, and each universe is infinite. Your theory is technically incorrect.

What if the world border divides up all of the different seeds

yeah that is what im saying

Also if it’s infinite, how does the sun go around it?

so if we both play on the same seed but on a different world we'll see each other?

no
every time you create a world it spawns you in a part of the world. thd only way 2 players can see eachother is when they are on the same world