Sharding is basically we have 5 instances of this area, so once we’re above a certain population, it’s going to be even split between those 5 instances.
Layering, you’re in an instance of the entire continent, and a second instance is only created if enough people are online. You stay in that instance unless you group with someone from a different one.
Except that it literally doesn’t unless you’ve memorized every single person on your server to be able to notice if they’re there or not, vs just not being online at the time. If me and you are questing together in the starting zone, we’ll keep running into each other because layering doesn’t work like you think it does.