In case some of you are curious, I'm afraid none of Dottore's sprites involve him having eyes or a very human-looking body(Yes you do get to see him shirtless hehe). This is due to my personal headcanon that Prime's original body is long gone. Yap alert! Art at the bottom.
So, Dottore has been confirmed to have been born at LEAST 500 years ago, after the Cataclysm. The Fatui were created around that time as well. The Tatarasuna incident happened 400 years ago, and by that time Dottore was already an established Harbinger(and the second, no less) so he's got to be quite old.
Dottore's entire character revolves around finding out how to twist himself into a divine shape and give up on his mortal shell. What could piss him off more than watching himself rot away with age?
Let's not forget he's definitely the type of perfectionist to hold profound disdain for himself, he's definitely seething from experiencing a basic part of human life. And then BAM, he meets this artifical puppet who's beautiful and perfect and unblemished.
So, as soon as he gets his hands on Scaramouche, I believe that's when he decides to create an artificial body for himself that he can use permanently. There is no Prime, there is only his consciousness spread out over twenty-four bodies as he starts making more Segments. I see him as not even having a brain made of flesh but rather akin to a hard drive, especially since his ingame model behind the mask doesn't seem to hold anything inside it's head. To me he's like a living ship of Theseus, where every part has been replaced and you can't even be sure there's anything left of him in there.
I also fully believe that Prime(so, the fleshy one) was super old before he went full cyborg, not old as in on the verge of dying but old enough to disguise himself as Escher who looked middle-aged. We'll know in the next AQ if he actually can shapeshift or if that was just a cool transition from hoyo 🙏
Anyways, here's a doodle of Prime back in the day. Hope this made sense, probably not since it's 7 am and I haven't slept.