They essentially wouldn’t get overly descriptive on what part is going where, more so they’re likely to just write it as vague actions so you can fill in the blanks with your own imagination.
At least that’s what I’ve noticed when seeing how people write 18+ Works for non-binary/gender fluid characters. Of course others who do decide to write them as having distinct male or female anatomy then they usually throw in disclaimer like “Character is non-binary but has male/female parts”
Hope that was helpful. I’m not really an expert but that’s what I would have to guess.