Some of Apple's code signing is open source (mostly in SecurityFramework). But not enough is open source to be able to build a modern `codesign`. The source you linked is ~10 years old and woefully out of date, for example!
I don't believe there are any Apple open source references for how notarization works (at least none before it was a public App Store Connect API).
There are even times when Apple's open source releases trail functionality they are shipping in macOS. For example, Apple recently added an alternative DER encoding of entitlements, which are expressed as a plist. I don't believe Apple ever published code for how the DER encoding works. Instead, we needed to use Apple's tooling as an oracle to incrementally derive the encoding.
I don't believe there are any Apple open source references for how notarization works (at least none before it was a public App Store Connect API).
There are even times when Apple's open source releases trail functionality they are shipping in macOS. For example, Apple recently added an alternative DER encoding of entitlements, which are expressed as a plist. I don't believe Apple ever published code for how the DER encoding works. Instead, we needed to use Apple's tooling as an oracle to incrementally derive the encoding.