Yep, it's not uncommon to close the doors as it stops the warm air from coming in.
The cold air in the cabin comes from the regular source that's used in flight, ie, the packs. There's three available, but you only ever use two on the ground because the APU can only make enough air to run two properly.
(They need 8 - 12psi to start running)
The air path into the cabin is -
- The APU inlet, near the tail
- Through the APU, then down the bottom of the fuselage to the belly where the packs sit.
- Pneumatic Air Conditioning Kits; PACKS, which cool the air down to about 3°C, then it's mixed with uncooled air to control the output temp.
- All that is blown into the cabin through a large number of ducts.
- It all eventually goes out the two outflow valves in the tail. They open and close as need be to control the cabin air pressure. On the ground they default to full open.
FWIW, when parked in a place like Jeddah, often the last thing the crew does before leaving the plane is to shut all the window shades to help keep the sunlight out; it all helps.
