How are Mac apps able to track a file's location?

macos has a special /.vol/ system mapped to the actual directory and files. The files and directories are accessible via /.vol/<device_id>/<inode_number>, regardless of where the files are on the file system.

It is a nice little system.

So, programs can for example get the inode number of /Users/jdoe/someFile.txt and then open it via /.vol/12345/6789 (in this case, device id is 12345 and inode number 6789). You then move /Users/jdoe/someFile.txt anywhere you want (on the same volume) and everything just works. You can even write a shell script that supports this magic.

ls -di <file> to get inode number.

$ ls -di /User/jdoe/someFile.txt
6789 /User/jdoe/someFile.txt

EDIT:

You use stat to get the id of the volume and inode number, according to linked answer as highlighted by IMSoP.

GetFileInfo /.vol/12345/6789 would return the current location of the file previously located in /Users/jdoe/someFile.txt.

See https://stackoverflow.com/questions/11951328/is-there-any-function-to-retrieve-the-path-associated-with-an-inode for more information.