How do I find my main water supply?

I’m a complete novice in plumbing so talk to me like a child. We have REALLY hard water in our area and the limescale build up is completely unmanageable. I want to get one of those magnetic devices that clip into the water supply pipe, primarily because it’s the cheapest option. If it doesn’t work I’ll try a better one.

However, I’m not sure which pipe to put it on. I guess the main cold water supply would be the best option. Problem is, my house has been completely renovated prior to me owning it so I don’t know where the cold water enters the house, and even if I did, I have a feeling it won’t be easily accessible now.

I’m just wondering does the water that goes to the combi boiler supply water to the rest of the house, and can I put the limescale remover device on one of those pipes?

Sorry if I’ve not been very clear, and thanks in advance!