[MichaelZuo]

It's likely not a single person understands the entire MacOS stack anymore, at least since Mavericks, all the pre-1985 old guard engineers having retired by then.

[Clamchop]

Interesting take given that a Mac today has virtually nothing in common with a Mac of the late 1990s, let alone 1984.

[MichaelZuo]

Says who?

[Kwpolska]

Mac OS X is a descendant of NeXTStep. It shares no code in common with classic Mac OS.

[MichaelZuo]

How is Mac OS Classic sharing lines of code, or not, even relevant to the question?

[s3p]

By definition. If they used the same code, they would be similar.

[MichaelZuo]

Yes but the opposite is not true, zero line of code shared does NOT necessarily imply zero or near zero similarity. Which is the case under discussion.