This is a good place to start.
But in all seriousness from the transgender people I’ve met both online and off. Their thought process can range from a lot of places.
Sometimes they knew all along, sometimes they didn’t, etc.
The only thing that upsets me is just how little support they get, not just medically but from family. I remember getting some dirty looks from a friend’s family when I refused to call him by his dead name, learning that your family doesn’t understand or outright refuses to is hard. They all just want support and to feel like they are actual people and not targets for whatever talking points of the week is.