I recently upgraded from an Android 7 phone (Moto G5) to an Android 9 phone (Moto G7). I like to back up all of my apps that I plan on using in the future, using a backup app like ML Manager, and installing them locally with a file browser, like FX. On my new phone, there are some APK backups that I can't install from a file browser; even if I actually download them though the Google Play Store, back them up, uninstall them, and then try to install that backup on the same phone, it won't work. Not every app does this, and at first, I thought it was only paid apps that were giving me problems, so I chalked it up to DRM, but a bunch of free apps won't work, either.
I've tried alternate methods of installation, too, like using a terminal emulator (see the screenshot, using the app Manual Camera as an example), and ADB on a PC, which results in an INSTALL_FAILED_VERIFICATION_FAILURE error. I've searched for this error and followed directions, like disabling Play Protect (which I never enabled on this phone, anyway), but nothing works. Is this just how Android is now? Did something change so that it's harder to install backups? I tried all of this on my wife's phone, too, which also runs Android 9, and I got the same errors. Any help would be appreciated, because I'm out of ideas. Thanks in advance!