My sister and I have Galaxy S4 and a Galaxy S3. Along the time of usage we have only downloaded common apps like social apps and games. We've never downloaded any system-related apps such as those used to root a phone. Thus, both phones are not rooted.
Is it possible for an app (before installing) to be in the form of .zip file instead of its usual .apk file (before installing)? (not the normal archive .zip file)
Does flashing a ROM mean putting a .zip file (with app inside it) in an external SD card, then pressing the Volume-Home-Power keys, later installing it?
Let's say I did all the steps in question 2 using the unrooted S3 and S4, will I successfully install the zipped app to the phones?
If question 3 fails, what other ways do I have to install the zip file? And I don't have access to the computer.
Those are the things that bother me. If there are things very wrong please correct me.