Someone educate me about vampire biology

I've always been curious of what an actual vampire anatomy would be like, if they actually exist. Is it even possible for them to transform as bats as well? How would that even work, with all of the body mass (presumably the mass of an average human) be compressed into a smaller creature? And in terms of internal organs too, how would their stomach work? How would their bloodstream work? I guess yeah someone could just create their own lore and rules in the universes of their oc vampires but I want to know how they'd actually be like if they were real HERE. So someone grossly educated please educate me