mac – How do APFS and the UNIX filesystem work in tandem on modern MacOS builds?

I am sure everyone here knows that Mac OS made the transition to its own, native filesystem, dubbed “APFS”, around the release of iOS 10.

However, it is also commonly known that behind the scenes Darwin (UNIX) is employed for standard libraries, filesystems, and other low-level system services that I do not care to mention.

My question is how Apple managed to create seemingly fluid interactions between the longstanding UNIX filesystem and the newer APFS (which, not to mention, has 64-bit inode numbers). Additionally, a high-level explanation on which level of abstraction the difference is made clear between the two. Is APFS a higher-level file management system or was it directly incorporated into the kernel?

Answers are much appreciated.