^ It is a bit of a complicated topic but I will try to break it down to the best of my understanding, apologies/warning in advance for the long comment and a bit of medical talk. Worth noting that this is my perspective and understanding, I am sure others would feel differently.
The term futanari taken literally just means "two forms"/"two parts", although it was generally used to refer to people that were androgynous or had both a dick and a vagina, what would at the time be known as a (fairly insensitive and largely outdated) term like hermaphrodite. However, the term generally was not synonymous with porn like it nowadays is (especially in the West), and mostly just meant a figure was "somewhere in-between" so to speak.
Eventually, around the late-20th century, some ero artists began to latch onto the idea for porn (some sources I have seen even specifically claim that transgender porn from the U.S. inspired a bunch of the earliest futanari comics, but I am not certain if it truly began the trend.) Fast forward several years (plus countless porn manga finding their way to the West) and the term has taken on a modern meaning that is almost completely sexual (at least over here, I do not know about its use in Japan but it is certainly more sexual than it once was.) That modern meaning largely falls into two categories: a person with fully-developed breasts, vagina, and penis, or a person with fully-developed breasts and a penis.
The former doesn't really have a real-life comparison, although it largely plays into an inaccurate stereotype of intersex people (the more respectful modern term for a number of differences linked to uncommon configurations of the sex chromosomes.) To my understanding, there are no common conditions which could cause someone to develop both a fully-formed penis and vagina, however there are some which result in under-development forms of both. Meanwhile, the latter form does have real-life equivalents, most prominently seen in two places: certain intersex people with Gynecomastia and trans women who have begun Hormone Replacement Therapy (HRT) but who have not undergone bottom surgery (surgery to replace their penis with a vagina, essentially.)
Regardless, whichever form is portrayed it still largely plays into the fetishization of both groups (seen as a hot fantasy by certain unpleasant people on the internet), because of how similar most portrayals of futanari are to said real-life perceptions. This is especially frustrating because it has led a large number of trans artists' drawings to be labelled futanari when they were simply trying to draw trans women in porn.
I'm not entirely sure how the terminology goes nowadays in Japan, unfortunately. It's possible futanari is still the common term for intersex people over there, it's possible some other term has been invented/borrowed. That said, I am a bit more familiar with Japanese terminology for trans people, and while I could see futanari potentially being used for a trans person it feels unlikely. More often you'll tend to see either "男の娘" (otokonoko, basically boygirl, although often used to describe cross-dressing too (which itself is sometimes confused as the same thing both here and there)) or I have seen some use "トランスジェンダー" (lit. transgender in katakana.) That said, in the West most trans women I know consider the term futa/futanari to be fairly offensive, comparable to a term like trap or sh*male (myself included.)
I am sure I glossed over some stuff here and there, since gender and public understanding thereof is a complicated subject even before accounting for differences in language, but that's what I mean both wrt when I talk about women with dicks that are not futa, and also my comments about the genre writ large.