Engine coolant expands when heated. Heated coolant needs a reservoir to come and go as needed.
Both. The reservoir is the overflow. But in the event your radiator is low, your reservoir will back it up.
It sounds like you might have a crack in your radiator or reservoir and will need to get it repaired or replaced.
You need to go get your car looked at by an auto repairman if you have a crack in your auto radiator. They can tell you if it needs replaced or just fixed.
A hot radiator releases excess coolant through the spring-loaded cap in to the reservoir, and when cooled the partial vacuum created by cooled radiator pulls it back in. A reservoir should be only about 1/4 full cold on a system that uses a radiator cap, some cars have a pressurized cap on reservoir and no cap on radiator
Put it back into your radiator. You have a head/ headgasket failure. You will need repair.
Not necessarily! Let it cool down then check the radiator water level and the water overflow reservoir tank. In a properly operating system the reservoir adds water as needed to the radiator when it cools down after you quit driving. #1. The radiator should be full of water and the reservoir tank should be 3/4 full of water. #2. At a certain temperature or with the "AC on" the radiator fan(s) should be sucking air over the radiator and cooling coils. So! Fill the radiator and the reservoir and when it overheats in idle see if the fans behind the radiator are operating.
Why does the radiator fill up the reservoir
Yes
The radiator does not have a cap, instead, the coolant system reservoir is also pressurized and the pressure cap is on the reservoir.
If the reservoir has a hose that travels to the top of the radiator, then it is filled with engine coolant. If the hose is not connected to the radiator, then it is most likely a windshield washer bottle.
The raiator is a type of system where all fluid is added to the reservoir instead of the radiator directly, look for the reservoir and you will have found your radiator cap.
My guess is that it should pur itself but double check with a parts store.