Shizuku is an open-source project you can find the source code and more details at https://github.com/RikkaApps/Shizuku.

The birth of Shizuku has two main purposes:
Provide a convenient way to use system APIs.
Convenient for the development of some apps that only requires adb permissions.

As a user how to use Shizuku?
Rooted devices: Start directly in Shizuku app.
Non-root devices: Follow the instructions (https://shizuku.rikka.app/guide/setup.html) it's not hard