Adding something the other answers didn't.
Original Magearna can be obtained in the Mystery Gift portion of Pokemon Home after the National Dex has been completed. This means every Pokemon from number 1-890 needs to be caught. It should be noted that the Pokemon exclusive to the Swsh expansion pass, (Kubfu, Urshifu, Regieleki, Regidrago, Calyrex, Spectrier, and Glastrier,) do not need to have been obtained.
Also, if you transfer any Pokemon from Pokemon Bank to Pokemon Home, and you have used a Gen 6 or 7 game to communicate with Pokemon Bank, any Pokemon, forms, shinies, and different language Pokedex Entries will all go into your Pokemon Home Pokedex. So, if you have, let's say, a completed National Dex in Gen 6, and you transfer one single Pokemon from that game to Pokemon Home, your Pokedex in Pokemon Home will now be filled out with everything that was in the Pokedex of that Gen 6 game.