How to convert an app/apk as system/user app, including split-apk apps

Background

Without root, I know that it's possible to install a single, normal APK using Intent (here).

Given root, I know that it's possible to install it using this .

This will install the app like a normal installation process, as a user-app.

There is also a way to install a split apk app (here), though for some reason I didn't succeed using it.

The problem

I want to know how to convert and/or install an app to be a system-installed-app, and back to be a user-app.

What I've found

I've found some very old ways to convert an existing app (or installing a new one) into a system app (here, for example or here), saying (in short) I should just move the APK file into /system/app/ path.

Thing is, this might have worked in the past, but now it's not. I think the reason for this is that the apps on /system/app/ exist in a different way now: each app has a folder with some files inside of it.

As for converting back to user app, I can't find it. Same goes for installing a split apk app to system app and convert it back to user app.

In fact, there are apps on the Play store (such as this one) that I remember that could convert other apps into system apps just fine (using root) - now can't do anything.

The questions

1. When did the method of installing/converting an app to system app worked? How come it doesn't work anymore?

2. What should be done installing/converting an app to system app, and back to user app, on both old and new Android versions?

3. Is the same thing possible for split-apk apps (example: AirBnb app) , or at least installed ones?

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