Magic the Gathering Arena is free to play and as such I think folks should be wary that it might fall into the trapping of more modern free to play predatory game leanings. For instance, as someone that played Hearthstone in it's original form all the way through and then left due to the game becoming more focused on needing to buy packs, Magic Arena feels more than fair if you want to play for free. Right away Magic drops you into a set of tutorial modes that familiarize you with the base game such that you can earn a pretty solid deck for each color. After that you can play quests to earn gold to buy packs and unlock dual color decks that offer you even more ways to play. Right now I have 7 decks they have given me for free and seem to be able to unlock more as I play the game. I imagine you could construct even more decks from the free cards you get too, but as a Magic newb I'm not sure i'd have great results with that yet.
If you want to checkout Magic Arena it recently came to mobile on iOS, Android and you can also play it on PC and Mac on the Epic Game Store. Not only can you unlock a hefty amount of free decks but right now you can use the code PLAYDnD to unlock 3 new packs from the Dungeons & Dragons meets Magic in the Forgotten Realms expansion.