Using a downloaded save file on an unmodified Switch is through standard means. While Nintendo provides an official method to transfer save data between two consoles via the "Manage Software" feature, this is for transferring your own saves, not for importing a downloaded one.
Sites like Project Pokémon host user-contributed save files that might feature a completed Pokédex or specific event Pokémon. Pokemon Scarlet Save File Download
The main file is the primary file containing your Trainer ID, Pokémon, inventory, and progression. Using a downloaded save file on an unmodified
Yes, absolutely. Using any save file from an unauthorized source violates Nintendo's Terms of Service. While not everyone gets caught, the risk of a is very real. This would lock your console out of all online services forever. and progression. Yes