This is yet another "it depends."
There's pre-assembly. The body parts are painted. Depending on make, there is partial assembly. Line design varies, so consider a pallet with main body, hood, trunk lid. Doors get fitted out, where and when depends on line.
Day of final assembly, frame goes on one end of line, the process starts - engine, drive-train, sub-assemblies, wiring harness, body join, seats, hook ups, detailing (all the little parts, etc.), fluids. Drive off.
Standard B-body GM, average I heard a