An APK is the file format Android uses to install apps. If an app is not available in your store, you can install it from an APK file by following these steps.
1. Download the APK file from a source you trust.
2. Open the file from your Downloads folder or your file manager.
3. If Android asks, allow your browser or file manager to install unknown apps. On recent versions this is a per-app setting under Settings, Apps, Special access, Install unknown apps.
4. Tap Install and wait for the installation to finish.
5. Turn the unknown-apps permission off again when you are done.
Before installing, check that the app name and developer match what you expect, and keep Google Play Protect turned on.