Can someone explain how to access the VST3 midi ports from my host or the juce demo host? I am confused about how all this works.

And how do I send a note to the port, and what happens to controller data?

Do you understand the fundamental concepts behind VST pluggins / VST Hosts?

Hint: look at the Demo VST Pluggin? - Esp how *it*  uses/generates MIDI data


{Or have I misunderstood, and you're actually trying to work-out how the Demo Host does it?}

I understand host and plugins.

Do you know about VST3? It does not follow the same rules. VST2 plugins only accept data from one midi port. VST3plugins provide unlimited midi ports to their hosts. The question is how to access them?