Hey there. I am not very much into mobile game development with UE4, but maybe I can give as much as I know about it.
UE4 have a good support for mobile ads, especially if you use some plugins like Bansh ads (free), yodo1 sdk (also free) or EasyAds Pro (paid).
As far as I know, natively UE4 onlu support Google Play services and Apple App store. Alternatively, I have read some documentations in past about Samsung sdk by which you can upload your app on their store. If you are looking for specific store, I am afraid you would need to look for their support guides.
If I get your question right, I think you are talking about system with higher config can run the game with better settings. There is a system for that in UE, you can use console commands that you can bind with UMG Widgets as events so whenever the user want to change the “quality settings” of the game those console commands run. However, It may not work for all features because some settings like distance field etc. are configured in Editor only so they’ll be included in game only when you build the game with those settings ON.
I have no knowledge about Xiamoi store, you should check out their support page or maybe write a mail to them.