Hey @meetever-chris, @Mikron, @swiderskip, @alfonso.moscato, @abiyani,
Happy to clear up the confusion here. The Fully Customizable SDKs are a new product that gives developers the ability to put audio and video streaming inside their own custom applications.
It is a video / audio service utilizing Zooms existing infrastructure. Think Video / Audio streaming as a Service. A good example would be a racing game where you can see and hear the friends you are playing with.
The Fully Customizable SDKs cannot join Zoom meetings or webinars, for that you will want to use the Client SDKs, which we are continuing to improve and allow further flexibility. 
You can see more about the differences between the two here:
Thanks,
Tommy