According to behind-the-scenes leaks (and a rare interview with the unnamed director who goes only by "Sully"), the production was filmed during an actual heatwave in the San Fernando Valley. The location was a dilapidated auto garage with no HVAC. The temperature inside reached 104°F (40°C).
Heat, Hard Work, and Summer Jones: A Deep Dive into "Sweltering Summer" blackedraw summer jones sweltering summer work