The Wikepedia information is accurate but confusing. Basically GM decided back in 1957 to standardize on one body only for each car and changing only the trim between the makes. The brands would retain their individual motors that made the car unique to the brand (at least up until GM changed that rule in 1972).
Full Bodied cars are called B-Bodies
Mid size was called the A-body but that platform morphed into the G-body
Compact cars were given their own platforms. X-body is a Nova, Camaro was an F-body; the Corvair was the original Y-body but GM recycled the name and today a Y-body is a Corvette. The old Vega Monza line was built on the H-body platform.