I've never used f-droid, but decide to try it on my (original) Pixel XL. I clicked "Download" and the file saved. I pulled down the top drawer and opened the fi

I've never used f-droid, but decide to try it on my (original) Pixel XL. I clicked "Download" and the file saved. I pulled down the top drawer and opened the file with "files". I clicked the APK and got a security warning with a link to the exact "settings" page where I change it. I clicked the security option off. I returned to files and clicked the APK again and it installed. This is very wordy and sounds a bit complicated, but it was simply a few extra taps and it installed. A pretty good balance of not too easy that a novice might do it by accident and still accessable.