Mineral sludge from tap water. A good radiator flushing will help and then use proper coolant to help keep the sludge building up and blocking the radiator.
Both. The reservoir is the overflow. But in the event your radiator is low, your reservoir will back it up.
Time to replace the head gasket.
You can use plain water to get sludge out of your car radiator. You can use Marvel Mystery oil to remove sludge from the oil pan.
Time to replace the head gasket.
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
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.