The Smart scan does appear to be a promotional tool to get you to sign up for more Norton products that will fix the “multiple dangerous/malicious” issues found in the scans. I would ignore that scan.
A Quick Scan is a scan that looks at the most vulnerable areas of your computer. Memory, Winodws folders, and drivers etc. The Full Scan does that plus it scans every file on your system. This scan can take quite a long time, so you want to run that while you have dinner or when your go to bed.
As long as you have run at least one Full System scan, the need for further scans is reduced because of the real time scanning done by the other elements of 360. Although a Quick Scan is automatically run after a LiveUpdate that updates any local malware definitions in your 360.