@Hyolobrika >How is it possible to "sideload" an app
It doesn't matter what signature is on the .apk, what matters is that the .apk has a signature - otherwise both `adb install` and the installer GUI will refuse to install it.

It's a pain in the ass if you've just edited the metadata of a large (free software) .apk, so Android will allow storing it a partition marked as non-internal and even though the editing is done, you can't install the thing without also signing it and the default signer is only really bundled with proprietary software.

>FOSS forks of Android then?
As typical of "FOSS", every last one of those forks is chock full of proprietary software.

There is only one free version of Android; https://replicant.us/