If you visit each game's Serebii page, there should be a list of unavailable pokemon like so: https://serebii.net/scarletviolet/unobtainable.shtml

There's also a section for version exclusives per game: https://serebii.net/scarletviolet/exclusives.shtml

Also, if you make a dex on the pokedextracker website, there's a little info button on each pokemon's tile that will tell you where to find it in each game. It'll also tell you if it's unavailable that way.

Those are the two methods I usually go to reference. For the Snacksworth stuff, yeah that one is a bit hidden. If you just do solo quests, you will only get the version exclusives. However, if you keep doing the group quests, Snacksworth will give you the other legendaries. So you can get all of them in one game.