trans girls are people who were previously thought to be boys and figured out that they are a girl later.

Also, I’m choosing to word it differently than you did and I want to explain why. For many trans people, it’s not so much a process of “turning into another gender”, but more a process of “figuring out what gender you actually are”. They might say that they have been a girl all along, but neither them nor people around them knew. So saying that they were a boy before (instead of saying they were thought to be a boy before) could be considered hurtful.