Hi Angus,
Hope this isn't too late for you! When a .HTML email is uploaded on Adobe Campaign Standard (ACS), there are two ways it might be imported: either using the in-built editor, or copy-pasted into the HTML view of the ACS editor.
You can upload your own HTML content to the Email Designer. If it hasn’t been created through Email Designer, it will load in compatibility mode which is designed to keep your original HTML but limits certain edition capabilities through the UI (such as the navigation tree and content fragments). Information about compatibility mode is here.
Or, you can build an email directly through the Email Designer. This lets you use the full functionality of Adobe Campaign Standard's email designer. You can find more information on that here, but the key takeaway is that it unlocks the features that are 'locked' for code safety in a normal editor.
You can also create HTML content, then convert it into an email designer template. It is recommended to do this modularly, i.e. a block at a time, rather than convert a whole HTML email template into one block (both for easier designing, but also for when building emails in that template in the future). You can find more information on that here.
Hope this helps!