An on-device store would be equivalent to Steam which first need to be installed on the device for example via the regular iOS App Store or sideloading or another already installed app store.
Sideloading would be downloading from a web page or secondary device or similar method.
Separate stores would be a decent middle-ground for my part when less tech-savvy people inevitably ask me "can I trust this app" I can reply with "are you about to install it from store X or Y or download elsewhere?". If the answer is "that store from Epic Games" I could sigh quietly and say "it's likely okay".