The macOS kernel (XNU) as well as the basic system underneath macOS (darwin) is open source.

https://github.com/apple-oss-distributions/xnu

I wouldn’t know whether that’s true, but this is partly hardware. That tends to make things more expensive, as, at some point, design bugs get really expensive both in time and in dollars, and chances are there were design bugs late in the process (e.g. because somebody published a new way of attacking a system that the latest design didn’t anticipate)

Also, Apple claims it was an effort spanning half a decade (https://security.apple.com/blog/memory-integrity-enforcement...), so depending on what you consider part of this (for example, do you include time spent on their secure memory allocator, on designing/implementing the ARM Memory Tagging Extension or Extended Memory Tagging Extension in the costs of this feature?),