I know a good bit about basic stellar evolution and structure. I know that lower-mass main sequence stars (excluding red dwarfs), like the Sun, have a radiative core surrounded by a radiative zone, which is in turn surrounded by the convective zone. I've read about how main sequence stars undergo a structure transition at about 1.2 solar masses, the start of the upper main sequence. At that point, the core becomes convective and the radiative and convective zones switch positions (or something like that). However, these were mostly very generalized descriptions I read, and I found only one very simple diagram (this one). I haven't found any detailed descriptions including diagrams, and (unless I overlooked something that was too technical for me to understand) I mostly found articles regarding things detailing certain specifics unrelated to what I was looking for.
So my questions are, in an upper main-sequence star (Sirius for instance), exactly how are the three main interior regions distributed? What percentage of such a star's radius do the regions take up? How does having a radiative zone where a lower MS star's convective zone would be affect its surface structure, appearance, and dynamics (e.g., granulation, starspots, flares & prominences, CMEs, magnetic field, etc.)?
Also, I know that red dwarfs have an entirely convective interior. However, I'm not sure exactly what mass does a star begin to have such a structure. I've read that it's about 0.25 solar masses. Is that right?