"I am looking for explanations that will help me build a conceptual understanding of macos UI development."
it depends on your learning style of course, but if you like lecture / course-style learning, i highly recommend paul hegarty's CS193P, it will definitely give you the in-depth, conceptual explanations you're describing.
while it is an iOS dev course (taught at stanford, lectures are freely offered online), the newest lectures are using swiftUI for UI, so pretty much all of it will translate to doing macOS dev.
paul is a cs prof at stanford and worked at NeXT and then on the objc lang many, many years ago, so he's been doing this a while and knows a thing or two :)
https://cs193p.sites.stanford.edu/