Unity is what they are using, I just know from seeing it in the browser code and researched it a little last year. https://unity.com/solutions/build-backend. Source: over 1 year ago

I never said that Unreal is bad for non BRs, just that it is very good for BRs. The amount of work they put into making Fortnite work as well as it does puts it way above anything else for large match based shooters. I agree that Unity's old multiplayer code was an absolute mess, and I would have previously suggested using a third party solution. I was saying that I've heard good things about their new multiplayer... Source: about 2 years ago

I remember a game on Linux called crayon physics deluxe, it was part of a humble bundle http://crayonphysics.com/. Source: over 2 years ago