-remain calm! It's hard when things get scary, but you need to be the adult in the situation.
-don't try to change your teen's mind. When has anyone ever changed someone's mind with facts? Not often.
It's OK to say that the idea of surgery is scary to you.
You can say that the existence of detransitioners makes you worried that even people who feel strongly about being trans sometimes change
When they come to you, probably with a request, then deal with that.
Then put your energy into other adults. Change the world!
Again, arguing rarely helps.