Let me try. My understanding of "internal" versus "conventional"(external) zoom
There are some zoom lenses that do not get longer when one zoom out because the zooming mechanism is constructed internally. An example is the Canon 70-200 f2.8.
But the Canon 100-400 zoom lengthens considerably when you zoom out, because the zooming mechanism was designed that way.
Of course there is a slight inherent advantage to have internal zooming, because the longer the lens, the less stable it is.
But I am unable to tell if the inherent mechanism of the internal versus external zooming makes for any real difference in the optical quality. There is no doubt that the 70-200 is sharper than the 100-400, but there are other optical considerations to explain the sharpness.
One of my favorite lens is the now out of production Angenieux 45-90mm. It is design to have an external zoom. But boy, is it sharp!