Coolant. Most likely DexCool.
The coolant goes into the reservoir only that says coolant. You don't put it in the radiator.
If the reservoir is low, ad a 50/50 mix of coolant and water. Or, buy it premixed and ad as is.
In the coolant reservoir in front of the engine.
Coolant is added to the plastic reservoir under the hood.
take the lid off the master cylinder reservoir and fill the reservoir completely and put the lid back on.
Add the coolant into the coolant reservoir. Be sure the engine is cool when you remove the cap off the reservoir.
In the coolant reservoir when the engine is cool.
Both. The reservoir is the overflow. But in the event your radiator is low, your reservoir will back it up.
Coolant for a 2006 VW Bettle is put into the coolant overflow tank. As it is needed, the radiator will automatically pull in additional coolant.
Fill the reservoir with G12 VW coolant only and it will go into the engine.
If there is no risk of freezing , but it will weaken the strength of the antifreeze / water coolant mixture
The only place available is the coolant reservoir.