What happens when you load multiple ROM files?

[SeriousComedian]

Do you get to choose which version of Pokemon you play, or do you play some composite/mashup of all versions of Pokemon?

[Munya]

You pick one to start in(Hoenn, Kanto, or Unova) and after a certain point in the story are given the ability to travel to a different region and play it.

[ThePrettyPetard]

no matther the region you start in you play the pokemmo version of it

[Akshit]

You can choose a region of your choice to begin in.

After getting 4 badges in a region, you can freely change region through port city (Kanto: Vermilion / Hoenn: Slateport / Unova: Castelia).

Though if you want to change region before that, can talk to your mother NPC before getting 2 badges in a specific region.

I would personally recommend having all the ROM files and exploring all the regions. All the best.