Question

If you breed a Pokemon from Legends Arceus with a Ditto, will it evolve to it's Hisuian form, say Goomy to H-Goodra?

Answer 1

If a Pokémon without a regional form evolves into a Pokémon with a regional form, it will evolve into the form native to the region it is currently in.

So if you have a Goomy, even if it came from Legends: Arceus, it will only evolve into Hisuian Sliggoo in Legends: Arceus, and it will evolve into Kalosian Sliggoo in any other game.

Source