So how does Keystone worlds work? And which ones are Keystone Worlds?
I’m sorry could you explain the lore of the canon worlds?
To put it very basically: Keystone worlds are kind of like the Capital Cities of their thematic source, they are not the only world in their thematic source, but they are essentially the ruling body of it.
The various worlds in the Magical Dimension exist to affect and stabilise the Universe, both magical and non. Within these worlds, there are various thematic elements which are shared across multiple worlds, and some thematic powers/sources have more worlds than others.
The purpose of multiple worlds, is redundancy. Backup systems.
'Smaller' or 'lesser' worlds exist as backup to the Keystone worlds, like extra 'oomph' in your pc to help the programs run smoother by reducing the strain.
When Domino was destroyed, other worlds like Pyros, Realm of Dragons, helped take up a percentage of the magical slack along side other worlds with Fire and Dragon themed sources. (Worlds that belonged to the same thematic network.)
Keystone worlds often have political influence over other worlds in their thematic alignment, but that's because their world is “doing the heavy lifting” so to speak.
All 6 of the Winx are from Keystone worlds.
Earth, despite it's importance, is not a Keystone world. (It is however a Tethering world which serves as a shortcut between the Magical Dimension and the Non-Magical universe.)