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About Time Machine local snapshots on Mac
Time Machine makes copies, each hour, of files that have changed. These local snapshots are stored on your computer’s internal drive, and are separate from backups stored on your backup disk. Local snapshots are a convenient way to retrieve earlier versions of your files, even when your normal backup disk isn’t available. You should always perform normal backups in case of a disaster.
Local snapshots are captured only on disks using Apple File System (APFS) when Back Up Automatically is selected in Time Machine preferences. On your Mac, choose Apple menu 
> System Preferences, then click Time Machine.