I’m not trying to be mean or overstep, I just want to understand..

So recently I had a friend open up to me, he told me he was trans. He said I knew I wasn’t judgmental and would be okay with knowing. Which I totally am. I honestly just thought he was gay even though he had a girlfriend for awhile, I just never said anything because I didn’t care in that way lol. And I didn’t want to make him feel uncomfortable or anything so I’m asking you guys, I just want to simply understand.. as far as it goes he said “he likes who he likes” and that he is trans so does that mean he is not fully a guy or just identifies as one? I’m not trying to come across rude or anything I love him and I’m not judgmental I just want to understand more so that I can be there for him. My intentions are good. So if anything doesn’t mind explaining I would appreciate it. Thank you guys! He has a girls name and changed it to a guys name. Also he looks like a guy. I had no idea before hand. I just thought he was gay. He has a mustache and everything.