Is diskutil, the command line tool on Mac, an open source program?
You already know that the open source code in macOS is here Apple Open Source. A significant section is the XNU kernel (some of it). There are also some libraries and utilities.
I am sure you are disappointed by the (lack of) breadth of what is available.
Specifically for diskutil, it is not there.
There is nowhere else that Apple publishes open source.
Most of macOS is not open source.