APFS technical documentation

Solution 1:

Apple File System Developer Documentation

Apple File System is a new, modern file system for iOS, macOS, tvOS, and watchOS. It is optimized for Flash/SSD storage and features strong encryption, copy-on-write metadata, space sharing, cloning for files and directories, snapshots, fast directory sizing, atomic safe-save primitives, and improved file system fundamentals.

See also Apple's File System Programming Guide.

Additionally, this blog APFS – A Backup Software Developer’s Perspective makes mention of APFS already being reverse engineered.

Solution 2:

Apple has released documentation for APFS. It's a little sparse, but most of the critical information should be there.

https://developer.apple.com/support/apple-file-system/Apple-File-System-Reference.pdf