short answer is : the "open" system where it pukes on the ground, it just establishes the amount contained in the system when hot. the excess is purged out to the ground. this type of system has only one seal on the cap.
the "closed" system has a cap with two seals. when the coolant is hot, the excess is released into the overflow can/bottle by pushing up on the lower seal on the cap that seals on the bottom of the cap neck. when it cools, the same seal lifts off the bottom of the fill neck, and vacuum draws the coolant back into the system without any air. after a couple of cycles, the radiator and engine is full of coolant without air in the system, and will remain as such.
the overflow can come into the reservoir from either the bottom or top, but the tube must end at almost the bottom [about 1/2" or so] of the tank to remain effective not drawing in any air during the cool down process.
the reservoir is where it's vented, either through a small hole in the cap [usually found on the fancy metal ones] or the sloppy fit of the cap to the bottle. [factory plastic reservoir]
this prevents the container from collapsing while still allowing the coolant to return to the radiator.
hope this short answer helps.