I'm an experienced Salesforce developer who build countless AppExchange apps in the last years. But I am absolutely new to Marketing Cloud. I know what it is and how it is used but have no idea on how to develop for it and on it.
I would like to build standalone or integration apps that connect to external systems (e.g. media content providers for Content Builder).
I know there are some APIs and proprietary languages, but I even struggled with step #1: getting a Marketing Cloud Developer Org.