Below you can see the overview of the entire Marine Corps Base Quantico. (Gray in the non-satellite or “after” map view)
“Quantico mainside” is to the east of I-95, and contains Officer Candidates School and the bulk of the rest of the support staff and other units at MCB Quantico.
The Basic School is located to the west amidst the large training areas. The main TBS location is called Camp Barrett, and you can download that map here: