While the more traditional speech patients get therapy for something that is the result of a birth defect/trauma/stroke (aka not something they choose lol) SLPs can help trans patients with how they speak/sound on an unconscious level through rigorous exercises/training, which is what they need to do if they don’t want to ruin their voices by straining them (such as by putting on a fake “sissy” voice). While there are some surgeries to supposedly achieve these goals overnight, they are typically viewed in the SLP as bullshit that will more than likely cause permanent vocal damage, aka lots of risk little reward.
The main areas the SLP focuses on are what the general public would perceive/“clock” as stereotypical male/female voice characteristics. For instance, as some people have mentioned already, tone, resonance, pitch, as well as pragmatic speech features all work together to signify a male/female voice. SLPs work with trans patients by having them manipulate their pitch, tone, etc to sound more stereotypically male/female, which includes their voice, body language/non verbal communication, and other sounds such as coughing/laughing.
Some examples would be how a voice sounds (phonetics). A phonetic characteristic like pitch is typically higher in woman (165-225 Hz) and lower in men (85-160 Hz). Another, resonance, results from the length of the vocal cords, and in men they are usually longer, resulting in a deeper/more baritone voice. Pragmatic features are the next level of speech. They are basically how we communicate socially.
All of this seamlessly works together to influence how we speak, and thus how we are understood. And there are gender differences. A few, off the top of my head: women have a tendency to do something called “upspeak”, which is when the pitch rises at the end of an utterance, something we all use when, for example, asking a question. However, women tend to use it more. Men tend to be more literal/“informational only” when they speak. These, on top of obvious differences in pitch/tone etc all work together to paint a picture of the gender/background of the speaker for the listener. There are obviously many, many examples of real men/women who do not sound/speak as what we would consider typically male/female, yet we still accurately perceive their gender. If a woman has an uncharacteristically deep voice, there are other semantic/pragmatic clues that we can use to detect gender.
While there are some trans people that have managed to effectively “mask” their voice enough to not be “outed”, it’s not very common. No matter the amount of surgeries, makeup, and other tools they use, as soon as they open their mouth we can instantly tell if it’s a man or woman speaking. Speech therapy can certainly help, but it’s something that is very, very difficult to change. Language usage results from teamwork by the brain and body, through biology and social conditioning/experience, and most of the people we talk about here are not surrounded by women, do not have female biology, nor do they have a real, true idea of what being a woman is, let alone what women sound/speak like.