You absolutely can install Linux apps. If you boot into the desktop mode there’s even an app store, but you can install stuff you download from the Internet or with a package manager.

Android apps are not gonna work. While the lowest level of Android is in fact Linux there’s a lot more running on top and android apps are built to run at that higher level.

You could probably emulate android, but I personally wouldn’t.

You can install Flatpak apps from the Discover store easily in Desktop Mode.

If you want to install system utilities you can disable read-only mode and use pacman (and paru, yay, etc. for the AUR) like normal Arch Linux.

Android apps are their own problem - in theory you can do it with Waydroid though.

It runs linux under the hood yes, KDE Plasma I think. You can install native linux apps etc yes. Just boot to the desktop. There is the discover app store which has a bunch of native apps listed in there.

For android, you may need some sort of wrapper or program to make that happen idk.