How do people get Pokemon from recent generations in older games?

Question

I've seen some Pokemon videos on youtube where their playing an older game, like ruby sapphire or black and white, and they have Pokemon from newer games in them with full sprite work and the whole kit and kaboodle. Like there's mimikyu and gholdengo, seemingly the entire national dex. Is there a mod out there for this or is this something commissioned for a youtube video?

Comments

What you’ve been seeing are called “romhacks”, basically custom fanmade games. They aren’t official Pokemon games.

The details of it we can only speak about limited due to it being outside of the scope of the website

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Are any of these good answers for this question? I just searched "rom hacks pokemon" and clicked the first few results.
https://www.pokeharbor.com/
https://www.reddit.com/r/PokemonROMhacks/
https://www.pokecommunity.com/forums/rom-hacks-showcase.184/
https://www.pokemoncoders.com/

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I think the above comments put together suffice as answer, this isn't asking anything more specific

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i'll let sumwun post the answer then as they did the digging. feel free to copy-paste my comment, sumwun