Does Time Machine copy everything all over every time

macOS uses what are called "Hard Links" between files.

Think of a directory entry like an address book. It contains information to contact a person. It DOES NOT contain the person, just a pointer. Multiple people can have address book entries for that same person. There is still only 1 person.

Hard Links are just additional directory entries that point to the same physical file.

Time Machine will either copy the modified or new file into the new backup tree, or it will put a Hard Link in the new backup tree if the file has not changed.

So while it looks like you have 12 copies of a single file in different Time Machine backup generations, but if the file has not changed, it is just multiple directories pointing to the same unmodified file.

No it checks what was involved in the last backup or previous backups and then any files/folders that have

been changed/ updated recently are then backed up. This saves space on your time machine backup drive.

Hard Link because a standard part of Unix based file systems back in the '60's (aka 1960's) with the very early Unix versions at Bell Labs. Directories only ever contained a name and a pointer to the file's internal metadata 'inode'. The internal metadata inode contained a filed called 'nlink' which is a reference count for each directory that has a pointer to this file. When a directory entry is deleted, the nlink count is decremented. When the nlink count goes to zero, then the storage for the file is deleted, and the metadata inode structure is put on a free list to be used for a new file sometime in the future.

The problem with the nlink reference count is, it does not know what directories contain pointers to the file's inode. To find that information, you would have to search the entire file system tree looking at each and every directory checking to see if the directory entry points to the file you are interested in.

What I'm trying to say is that because the Time Machine developers just piggy backed on existing Unix file system technology, they did not have an easy way to tell if any give Time Machine backed up file is the original or is a hard link. And remember ALL files, even the original, use a Hard Link from the directory to the inode. There is not difference and as long as 1 Hard Link still exists, the file exists.