Sideloading, or the process of pushing an app to your Android device from your computer instead of downloading it from the Play Store, is the only way to install apps on some modified Android devices. It primarily allows developers to test their apps before publishing them. Because of this, you’ll need Android developer tools on your computer to sideload an app. The primary one is Google’s Android Debug Bridge (ADB).
Android 101: How to Sideload or Install Android Applications
ADB is used by developers to transfer data between a computer and an Android device. It allows a developer, or just someone who wants to tinker with their Android device, to control their phone from a computer, transferring files, installing apps, and even running a console on the device with root privileges.
Google makes ADB available for free to everyone. You can download it directly from them and install it on your computer.
Open your browser and download Google's ADB.