Why it said apk.zip when I downloaded it
That's an Android bug, some users have reported this issue. Just download the apk on your computer and then transfer to your phone :)
You can use zarchiver and remove the "zip" and turn it to "apk."
Some phones auto convert the APK when download it, there are some fixes around. You can download from a PC and transfer to your phone or try to rename the file format from .zip to .apk
