My understanding is that Avast (and many other AVs) quick scan, scans memory, threads and system files and full scan goes to other partitions/disks and scan every file stored there, even .zip, .rar and other compression formats it can get into. That's why Full scan takes much longer. You can see exactly what it scans during the scanning process. Boot time scan is reserved for scanning and eventually cleaning infestation in system files that can not be changed during full system work because that would stop OS from working.