I hate to be the guy firing links at you - but it can get a complex thing and epic has written dozent of articles about it: Frequently Asked Questions - Unreal Engine EULA - Unreal Engine - there is also custom (studio) licensing and a dedicated mailadress for licensing questions. But in short, yes steam one time 100$ (per game) for building your store + a big number % of your revenue, some percent (look at the links, I think 3% or so) of your revenue after a decent revenue goes to epic (royalty) regardless where you release your game (!!), there is itch.io which is good, you can also publish to epic store of course or other indie stores. So in really short - if your game is free2play and doesn’t generate revenue (which I doubt) its only the fee from the store like steam 100$ one time. I’m not a epic staff, so don’t blame me if my information is wrong 
you can retag your question there is a dedicated category for licensing, there is the chance quite good a staff will have a look…