It says its 'headless' but I can make regular Umbraco headless anyway by returning Json from the controllers. So what really is the difference / advantage of Heratcore?

I am by no means an expert, but the upside of HeartCore vs regular Umbraco as far as I understand it is: - Built-in graphql api - A CDN with redundancy, region support etc that is not coupled to the Umbraco site So if you do composable architecture and just need the CMS with nothing else and need the content delivered in a reliable way it may be a good fit. On the other hand you can't develop on it yourself so it is purely a web platform for managing content with some apis you can use in your application to fetch content from. If you normally do monoliths where all business logic is within your umbraco site then its probably not the right fit.

Thanks " you can't develop on it yourself" Does that mean you can't add your own Controllers or any logic You just get raw data to MongoDB? I guess you can add webhooks which trigger functions, for example?

You do not get code access, so you have your online running umbraco site that you can manage content through, and it has a few additional things that are not normally on an umbraco site like api key functionality and a graphql playground. But you cannot install packages, and cannot touch any code. You will have to have some other application where you fetch data from the Heartcore site