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.
Questions:
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.