AutoSave and Backup are two separate functions, and they save files to two separate locations.
Backup saves a copy when you Save. If you don’t save, Dorico doesn’t store a backup.
AutoSave saves with the frequency you specify, but not to the backup folder. On Mac, AutoSaves are stored at ~/Library/Application Support/Steinberg/Dorico 3.5/AutoSave
When you close a project normally, or close Dorico normally, Dorico dumps the contents of the AutoSave folder in the Trash. When Dorico is launched, it scans to see if there’s anything in the AutoSave folder: if there is (because Dorico crashed or was forcibly quit), it asks if you want to “recover” (e.g. load those projects from the AutoSave folder).