I am also concerned with finding a way to backup Apple's HFS+ and APFS filesystems to some other *nix filesystem.  The problem is that they are very complicated, with each file being a complex data structure composed of a so-called "data fork," "resource form," and Finder metadata.  The standard backup tool, `rsync`, will not recognize these various components of your files. This is a very partial answer, according to what I have found so far. 

There is a project called `rsync+hfsmode` that will handle this properly, at least for HFS+, but it does it by creating two files on the backup drive, `filename`, containing the data fork, and `._filename` containing the resource fork and Finder metadata.  Furthermore, when copying back to an HFS+ disk, a second step is needed to reconstitute those two files into a proper HFS+ data structure.  You can see a more complete discussion at [the project page](https://quesera.com/reynhout/misc/rsync+hfsmode/).  I am not clear if this will work for APFS.

In looking for other solutions, it appears that `dar` (Disk Archiver) can also handle at least some of the unique characteristics of HFS+ filesystems.  According to their [Features page](http://dar.linux.free.fr/doc/Features.html).

> HFS+ family contains only one function : the birthtime. In addition to ctime, mtime and atime, dar can backup, compare and restore all four dates of a given inode (well, ctime is not possible to restore).

Wikipedia also has [the comment](https://en.wikipedia.org/wiki/Dar_(disk_archiver)#Features) that `dar`:

> Takes care of some filesystem specific attributes like Birthdate of HFS+ filesystem"

Whether this means they are also handing the data/resource forks and the Finder data properly, I am not clear, but I am supposing that is probably the case.

For the purposes of your question, you will in addition want to know how to represent all of this data in a usable way on some linux FS.  Clearly the Finder data is of no use, and you will have to lose the "birthtime" attribute, whatever that is, because that is not tracked in Linux.  What relevance the resource fork has, I am not clear.  I do know that there is some Mac software will only run on Apple filesystems.  (The one I've come across was image editing software, whose name I've forgotten.)

I welcome all additions and edits, as I need this answer myself.