General question about Windows 10 platforms.

The first computer I have ever used was 19 years ago and it was Windows XP, people say that it was one of the most stable windows of them all. The unity build for PC generates the kind of app that everyone was and is familiar with at that time. All the apps, now and then are mostly an archive containing the .exe and data files, or a setup installer that organize better the app files on the system, instead of unzipping an archive that contains randomly placed app files.

I remember in 2014 I bought a nokia smartphone, powered by Windows 8 mobile and then upgrading to windows 10 mobile. I then learned that, similar to android .apk files, windows mobile uses .xap or .appx and had some strange setup files called .appxbundle. I understood that they needed a new file type for windows mobile apps, however the fact that these XAP/APPX/APPXBUNDLE things were available to PC was confusing. In the normal exe world, every application had their own tools bar, menus, input fields, buttons, they all looked differently, for the apps you install from web. However the apps from Windows Store PC, all used the system theme color for the frame, all the buttons in all apps looked the same, all the input fields were identical and it seemed a very controlled environment. Now I think this is called the UWP(universal windows platform) framework. I tried creating an app for UWP and when I ran the program an empty window appeared, with the system color on the frame, and was exactly the type of app that you download from Windows store, so I concluded that the UWP = APPX. I have no idea how this framework works and the appx stuff so I will write some questions:

1. UWP == APPX?
2. Are all apps from Windows store built on UWP?
3. If 2 is true, complex games from store like forza horizon are build on this frameworks which seems limited in terms of windows store?
4. Windows store apps are meant to be seen from the perspective of someone willing just to use them. They feel like android apps. You click install and the rest is under the hood, you don't care where it is installing. This fits for Mobile operating systems like android, however for PC, since beginning the classing wizard setup asked you for location, and you could actually go there and check the data files for the app and see readme.txt which shows that you are meant to check the install folder and look at the app and how it works. However with Windows store apps, the system stores the apps in C:\Program Files\WindowsApps which is a folder that windows doesn't allow non-administrators to check which shows that they intended for the app dat to be accessed just by the apps code. This kind of apps don't fit the idea of a PC.
5. I have a steam app build with unity and exported as a regular "exe + data" game which is how steam games work. However If I export in unity for appx, can I send the appx to steam and when someone downloads the steam app, the appxbundle instalation starts?
6. Can you export in unity for UWP and/or APPX?
7. In my opinion this creates a paradox. APPX for windows 10 are like APK for android: They recommend to install them just by the official source(Windows Store for APPX and Google Play for APK), however 99% of PC apps are third party apps downloaded from third party sources so why they recommend not to install appxbundles from third party websites, however if you are installing a msi setup, they say nothing(just if the defender sees the app as a thread)?