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

Try the ENDALGO app to find and host your pickup games. You can collect RSVPs and accept payments. Organizing groups is 100% free, unlike Meetup or any other apps. I've been using it for over a year. It may start to slow, but you'll find people when you keep your event listed for a few months. Source: 12 months ago