On Android 8.1,
If I go to "Settings" > "Apps & Notifications" > "Advanced" > "Special App Access" > "Install unknown apps", what determines which app are in this list ?
On my phone, I see :
Chrome, Drive, Files, GMail, Messanger, WhatsApp.
Yet I have a lot more apps installed and running.
It lists apps that have the permission to install other apps. The permission is found on the apps' manifest. For example, a file manager can install an apk, same case with a browser or Whatsapp if a contact sends an apk to you.
They are labelled Unknown apps because the apps are not installed via Google Play Store, thus not trusted by Google (that does not mean that there is something wrong with the file).
From @Andrew T's comment, the permission is REQUEST_INSTALL_PACKAGES that:
Allows an application to request installing packages. Apps targeting APIs greater than 25 must hold this permission in order to use Intent.ACTION_INSTALL_PACKAGE.
Note: API 25 is for Android 8.
Q & A