Officially MTGA is developed in Unity and therefore can be easily ported to Mac, Linux, tablets and phones in the foreseeable future. That’s yet to come as the developers would like to iron out the game on PC first. Unofficially, you can jury-rig your devices and work around the limitations.
For Mac:
Download Windows and use Bootcamp to install it. Press and hold the Option key to switch between MacOS and Windows during boot up. Once in the game, you may need to tweak the video screen resolution. This setup had been used since the beginning of Open Beta by some players. MTGA is compatible with Win7 and above.
Alternatively you could try NVIDIA GeForceNow or Parsec. There's also a guide here
For Linux:
This discussion may be a starting point.
For tablets and phones using iOS:
Use Chrome Remote Desktop/Teamviewer for remote desktop access or any other alternative. Currently that’s the only known way to play MTGA on those devices.
For tablets and phones using Android:
Use Steamlink for remote desktop access or any other alternative. Currently that’s the only known way to play MTGA on those devices.
For especially gifted you may even jury-rig it for Nintendo switch
Searching the subreddit for the device of your choice can yield the most up to date information on how to play Arena on the device you want.
Now that we’ve very briefly discussed the differences between the current offerings, let’s have a closer look at how MTGA may be played and the structure of available formats