Android apps have a single APK file where I can easily import from my computer to my device and install it. I believe that iOS has ipa files. PA files are programs written for Apple iOS devices such as iPhone, iPod Touch or iPad. Such files are downloaded from the Apple iTunes App Store and can be transferred to an iOS device when it is connected to the computer.
APK files are App packages for Android, not iOS.
They WILL NOT run on an iPhone. NEVER.
The coding is different, and will have to be re-written and re-compiled in XCode, and then you will need to transfer it to a jailbroken iPhone. There are some converter programs these days, but they are bad, you need to re-write it yourself to ensure no bugs.
Installer.app is a free iOS software initially developed by NullRiver. This app allowed any iPhone user to install third-party apps into the iPhone Application folder where other native apps reside.