. If you see "Pokémon Diamond GBA" online, it is almost certainly a —a fan-modified version of an older GBA game like

That's it! The game should start, and you can begin your journey in the Sinnoh region.

Legally, you must extract a clean ROM file of Pokémon FireRed or Pokémon Emerald from a physical cartridge you own.

No. To be absolutely clear:

: An older fan project attempting to map the Sinnoh story onto a GBA file.