RetriX is a UWP native game console emulator based on Libretro.
Supports
It is free, open source and best of all you can get it on your phone right now - check the homepage, no need to worry about store bans :)
See it in action also on:
It's a packaging format. Like modern .msi
. It doesn't have any connection with the Windows Store, except for that the Store enforces its apps to be packaged as .appx. But you can freely choose whether you want to publish your app as .exe, .msi or .appx
For example Retrix is distributed as an .appx installer. I've personally repackaged Thunderbird to test it, works perfectly.
the sideloading works the same as for example on Android.
It installs completely without the store (that's not to say that the Store might not also use the same installer mechanism as sideloading in the background) but the app still has a unique Identifier so the Store (if active) will identify it and be able to update it should a newer version be available in the Store.
Of course if it is not available in the Store at all like https://www.retrix.me/ then it's just a fast and easy way to install a program - like a well made .exe setup.