Create Cubase Extensions - how?

I’m a developer who’s interested in creating an extension in Cubase for an external controller, but i can’t seem to find any information on the subject.

I see there’s SDK’s for VST components, but nothing for extensions, are they within the VST SDKs? - can anyone guide me as to where to find out the information to get started?

Many thanks.

There’s a developer forum at https://sdk.steinberg.net