You can play on mobile but the iOS slots are limited and full (10,000 in total). So, iOS is not available at the moment. Android is fully available.

They have their own sidechain known as Ronin. You will have to move your stuff from your metamask your Ronin wallet, and vice versa, through the Bridge. If you look at the etherscan's gas tracker, Ronin Bridge is usually top 5 in gas consumption, that's just how crazy this game is at the moment. It's just one game yet they are burning gas at the same rate as the likes of Uniswap and etc.