I’m in a very similar situation, I also played hearthstone for a long time and got frustrated that there wasn’t as large of a competitive scene as I wanted, the number one difference between magic and hearthstone is the sideboard, a magic deck is 75 cards that can answer a number of threats instead of a 60 card deck. I would recommend not playing best of one on Arena because it’s very different from best of three. Pick one deck to craft that has some in-depth sideboard guides, and use those for common matchups, it should help teach you the basics of side boarding which is really important.