it's not a mod, but regarding the assets owned by EA, it's using the license granted by the 'C&C Franchise Modding Guidelines' [0], so that's why I used that word
You don’t. I just installed it yesterday on a new Linux laptop. What’s interesting about openra is you can run multiple copies at the same time on the same machine to play against yourself for fun
[0]https://www.openra.net/download/