If I may make an observation of the way my mind naturally imposes a context to the HTP discussion:
this forum is about Canon cameras and the way they work. With few exceptions the reason people come here is to exchange information about Canon cameras, present and future to be sure. Therefore, when I read the expression "the mechanism of HTP" the primary context is, quite naturally and automatically, "what happens inside my camera when I enable HTP". It means "how are the principles of headroom and ISO gain applied inside my camera". It means "what effect does enabling HTP have on the way my camera behaves".
So with the behavior of Canon Cameras paramount to the discussion, I don't see why it is so difficult to think of Canons implimentation of HTP in these terms:
Set your Canon camera up to take a picture at ISO 400.
apply cyanoacrylate to the ISO dial -- just not to the electronic gain structure inside the camera
take the picture
pause again. look at the dial. whew it is still set to 400.
look at the ISO dial again. yep, the cyanoacrylate is still there. ISO still set to 400
take another picture. how did the camera behave differently under the influence of the HTP setting?
THAT, to me, is the "mechanism of HTP". Reverting back to the general application of base ISO headroom, the special case of ISO 100, the benefits or characteristics of non-Canon cameras or sensors -- none of that helps describe what Canon cameras do under the vast majority of use cases when HTP is enabled.
Furthermore, a discussion on the differences between Canon and Sony sensors (and the supporting electronics), as regards their respective behaviors at high levels of light (near saturation) and how to advantage each sensor to the given situation is engaging to be sure ,but doesn't help us understand how Canon Cameras behave differently when HTP is enabled, compared to the way they behave when HTP is not enabled.