1991 Volkswagen Jetta Radiator/Coolant leak

Heater problem
1991 Volkswagen Jetta 4 cyl Two Wheel Drive Manual

In my 1991 Volkswagen Jetta Carat, the coolant reservoir does not maintain a stable level of coolant and I found that coolant is leaking somewhere and when the engine heats up makes a kind of sweet smell. I've checked the reservoir and hoses and found no leaks, but (I'm clearly not a car guy, bear with me) where the coolant lines meet at a junction on the side of the engine, there is a round opening on one side of the junction where the coolant is freely leaking from, but there doesn't seem to be any disconnected hoses anywhere.
So what is that junction and what would be recommended to repair it?