Arena keeps crashing for mobile, it could crash during a game, or during deckbuilding.

Step 1 - open MTG Arena on mobile app.
Step 2 - play some games.
Step 3 - repeat Step 2 until crash. It freezes, and if you start it back up, you can continue and finish the match, chat won't be available, and the game screen goes black after the match.

For the deck building crash,
Step 1 - Start mtg Arena
Step 2 - start deck building
Step 3 - continue on deck building until it freezes. If you close it, when you open it back up, it may let you continue deck building, or freeze immediately, and this time, if you open it back up, your progress will be erased.