Hey Chris, sounds like Gamith has given a pretty thorough rundown. The cooling system on your 2006 2.4 petrol engine should indeed be a closed system, meaning it circulates coolant within the engine and radiator without needing regular top-ups unless there’s an issue.
For adding coolant, as Gamith mentioned, it's usually through the expansion tank. Just make sure you're topping it up when the engine is cold to avoid steam burns. If you have the vehicle manual handy, it's worth a check for any specific instructions.
Let us know how you get on, and if everything checks out okay. It's always good to hear how these things turn out!