Seems like firmlinks are like a union file system, such as overlayfs. A lower dir placed on a read-only file system, and merged with an upper dir on a read-writ

Seems like firmlinks are like a union file system, such as overlayfs. A lower dir placed on a read-only file system, and merged with an upper dir on a read-write file system. It appears as if they were one directory. Overlayfs will do an efficient (reflink) copy-up on a supporting filesystem, such as XFS or Btrfs. Both layers need to be on the same file system to do the reflink copy. The lower dir can be made read-only by VFS read-only bind mount option; or on Btrfs either a read-only snapshot or setting read-only property on a subvolume. But it sounds like APFS firmlinks institute some limitations, like you probably can't modify files on the read-only layer via copy-up to the read-write upper, rather just add new/unrelated items.