Unfortunately, I found that resources on Cocoa are very lacking, you have to search all day to get some trivial thing to do. The best resources you have would be WWDC videos and transcripts[0]. If the subject you’re looking for is older than a few years, the documentation archive[1] might have detailed, good documentation with concepts and sample code. Some more obscure features will require you search all day, find a few blog posts from ~10 years ago, and munge them enough to make something workable.
The best resource of all was the help of more experienced Cocoa devs – the AppKit Abusers Slack [2] was very helpful and pointed out problems when I had problems. Very grateful for them.
[1]: https://developer.apple.com/library/archive/navigation/