The bottom circle is the radiator cap. Coolant goes in there too (the same coolant that goes into the top circle reservoir).

The reservoir and radiator are connected, so coolant can flow between them. So normally, you never need to open the radiator cap, you just check the reservoir. But if the reservoir ever gets very low, the radiator might be low too. And it takes a long time (days) for much coolant to flow from the reservoir to the radiator. So it might be a good idea, in your case, to open the radiator cap (but only do this when the engine is cold), just to check the radiator is full to the brim with coolant. And if it isn’t full to the brim, top up the radiator, as well as the reservoir. Then start looking for leaks.

The metal cap is is on the radiator and maintains pressure in the cooling system. This should be “full the brim” under that cap. The black plastic cap is the overflow bottle. The idea is that when the coolant heats up and expands, it overflows from the radiator into the coolant bottle. When it cools down and contracts, it sucks the coolant back. This does not always work perfectly, however. So check the radiator (metal cap) as well as the coolant bottle. If the radiator is full but the coolant bottle is low, you are safe in the short term. However, if the radiator is low, it doesn’t matter if the coolant bottle is full… you could end up with problems due to not having enough coolant.