An APK file is designed for Android. On iPhone, access is usually through the App Store when an app is available, or through the mobile site in Safari.

iPhone guide: why an APK cannot be installed on iOS and which alternatives to use to access the mobile service.
iOS does not install Android APK files. Downloading an APK on iPhone therefore does not let you install the app.
Avoid sites that promise a direct APK installation on iPhone: they can be misleading or risky.
Check the App Store for your region and the operator's official information.
If the app is not available, use the mobile site with Safari and enable only the options permitted in your country.