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The Android Debug Bridge (ADB) is a command-line tool that can be used to install, uninstall, and debug applications. It also may be used to transfer files and access a device shell. It also is a necessary component for tools such as scrcpy or Vysor.
The stock Keitais do not come with Google Play Services, or the Play Store. Also, the phones do not come with any built in way to install apps.
Keitais bought from online sellers will sometimes preload the phones with Aurora Store or F-Droid. Otherwise, you will need to use ADB in order to install apps.
Setup
First you will need to enable USB Debugging on the phone. Go into Settings>About Phone, in here click Build Number 5 times (It will say “You are now a Developer!”). You will now have access to Developer options which will be in Settings>More Settings>Dev Options. In here find USB Debugging and enable it. Now you may make use of ADB.