The state of the docs is what got me to dump all of the links I'd been collecting to useful, but hard to find docs in to a small website -- https://www.iosdev.recipes/
Apple has been doing a great job in some aspects at documenting more and updating things gradually, but at the same time, has been hiding more and more of the "old" docs and it's a shame since many of them are still correct, relevant and useful.
Some things are incredibly useful and well organized, but completely blocked from indexing, like the documentation for xcconfig files: https://help.apple.com/xcode/mac/current/#/dev745c5c974
Apple has been doing a great job in some aspects at documenting more and updating things gradually, but at the same time, has been hiding more and more of the "old" docs and it's a shame since many of them are still correct, relevant and useful.