How does Magisk work?

[Magisk](https://github.com/topjohnwu/Magisk) is known as a “systemless” root method. It’s essentially a way to modify the system without actually modifying it. Modifications are stored safely in the boot partition instead of modifying the real system files. I have looked around but did not find a sufficient explanation as to how it actually works. How is the root access gained and maintained? What exactly is the role of the boot partition and if it integrates with the system partition how it does it? A really detailed description of how it works lacks from everywhere I searched so it would be really highly appreciated.