Apple says that this space is hidden and not accessible to users, but I don't think it's like that. It must be stored some where, so where could this be stored?
Mac – Time Machine local snapshots stored in High Sierra
high sierratime-machine
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The actual data is stored in a hidden directory called .MobileBackups at the root of the volume.
There is a special virtual filesystem type called mtmfs which translates the raw data in these hidden directories into a virtual Time Machine drive which is automatically mounted on /Volumes/MobileBackups
This mechanism of storing local backups existed on HFS format file systems and is no longer how things work on APFS Macs. Snapshots now are built in to the filesystem as opposed to needing to make “shadow copies” of files in a new location, the “shadow copies” are of the whole disk state.
It seems that local snapshots are enabled automatically in High Sierra.
The text in the red box translates to "local snapshots, as long as disk space is available".
I didn't find something from Appe on this, but this blogpost comes to the same conclusion.
Best Answer
Local snapshots are stored in
/Volumes/com.apple.TimeMachine.localsnapshots