[DISCUSSION] How did new regions work in the Pokemon world?

[Tenebris_Kane]

I like Serebii's version of it, they made a map which they call Pokéarth here although it looks like it is missing alola currently

I don't really understand the whole 'new Pokémon' thing in each region as when they had flying Pokémon and water transport for soo long they should have been to each of the regions