It’s a term few decades old, and means transferring files between local devices.
You download the app on your pc, you sideload it to your local device (your phone) using ‘adb sideload file.apk’, and you use that installed app to upload pictures of your mom.
RunawayFixer@lemmy.world 13 hours ago
I usually download apk directly to my android devices and install from there, no pc or other device is needed. So your whenever is for me almost never.