This is nothing bruh
I've got 6 EDH (commander) decks on paper here that will make you cry in agony
But yeah this game is awesome and seriously complicated in equal measure
In general, 'own outside the game' is referring to a sideboard selection. Even if its non-competitive, you'd find some sort of limitation, because otherwise you'd have access to virtually every card. On paper, playgroups have rules for this. In some formats (like Commander) there is no sideboard at all and cards like that would be useless.
With the Strixhaven set release, the sideboard has become even more limited in size if you also use Lessons, as these also end up in your sideboard which is always capped at 15 cards. This is relevant for the current rotation in Standard (and lessons are pretty neat ways to get card advantage).
Additionally, 'cards you own' are not referred to as your 'Library'. The library is your actual deck of cards you're using - specifically, the stack you've not drawn yet.
If you want some catch-all hints and tips to get you started with understanding the game on a deeper level, give a shout, happy to provide
One big basic tip on card rules:
Has every card and with rulings if something was not clear in actual play.
mtg.fandom.com
Has all the rules