system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do transgender sexual organs function?
gpt
Both men and women have a urethra, which is a tube that connects to the bladder. Male to female reassignment surgery snips and reroutes the tube to a urethral opening like what a woman has, and creates a separate orifice for the vaginal canal. Female to male reassignment surgery extends/stretches the urinal canal with a tube which ends at the tip of the penis. There isn't much need to reroute a second tube into the urethra like how a cisgendered male's urethra is connected both to the bladder and testes, because a FtM man cannot produce ejaculate so the second "tube" connecting is pointless.

Vaginas of MtF women work pretty much the same way as natural vaginas from an intercourse perspective. Reproductively speaking there's no uterus or ovaries so MtF women are universally infertile, but in terms of the mechanics of how intercourse is performed it's the same basic idea, there's an orifice which is stimulated by penetration and external stimulation.

FtM men's penises are generally non-functional sexually, which is part of why FtM genital reassignment is less common. Surgeons can construct a reasonable facsimile for a penis, and it can be sexually sensitive to the point that orgasm is possible, but we don't have the ability to surgically create a penis which can achieve natural erections upon arousal. The only existing solution is to insert a prosthetic device which can go limp or stiff depending on user control, so, the patient essentially has a "boner button" that they can turn on and off as needed.