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IIRC (from a TV show I saw 15+ years ago, before How It's Made I think), GM uses a burp funnel equivalent and they pull a vacuum in the coolant system with the overflow hose (which goes into the overflow tank from the radiator neck hose) until the engine reaches temperature at the factory...

Coolant getting pushed out usually signifies a pressure introduction in the cooling system. If you smell something that's similar to exhaust from the radiator cap opening with the engine running, you may have a blown head gasket. It goes without saying that you should start with a cool engine...