The migration from HFS to APFS does not move your files at all. It leaves the files exactly where they are on the disk. Instead of moving the files, it writes new the new APFS metadata onto unused areas of your disk. When it's done with that, it changes the superblock header to point to the new APFS metadata. Then it marks the old HFS metadata as blank space. Your files are not moved or copied.

By doing it this way, they make the dangerous part of the process as small as possible. They actually did a dry-run to collect success/failure metrics before APFS was released to iOS devices:

https://www.macobserver.com/analysis/apple-dry-run-apfs-prio...