Here's a thing about High Sierra Mac laptops with APFS (SSD): you don't need an external drive to perform a Time Machine restoration.
I rebooted my MBA via ⌘R at restart & ran the First Aid aspect of DiskUtility. Mention was made of the existence of nine local snapshots, 6 from the previous day & one each for the previous three days.
-- Each hour, Time Machine takes a snapshot of the whole system and saves it somewhere (only if you have plenty of drive space). When a backup is made to an external drive, these local snapshots are replicated on the external backup. I don't know if they are then cleared from the Mac.
A snapshot is automatically made as the initial stage of a macOS system update.
Now for the interesting part. Under the Restore from Time Machine section, each of these snapshots is recognised as a source for a complete Time Machine restoration. You can also restore missing files if they were deleted any time after a snapshot was taken.
I don't know if that snapshot process works on a hard drive or Fusion drive Mac, though. I'm not even sure if desktop Macs use the snapshot process at all.