My brother, 21 yo, is like this. He’s ~180 cm tall and has never weighted more than 61 kg (during the 2020 covid lockdown, he made a big deal out of this since it was the first time he passed the 60 kg mark; after the lockdown was over he immediately dropped back to his perpetual weight of ~55kg). I live with him so I know his eating habits: he eats all the time, whatever he wants, whenever, how much he wants and simply doesn’t put on any weight. Is very energetic, no sick look / demeanour. It’s just fascinating to me cause I’m not like that at all, I have to watch what / how much / when I eat.
this is actually misinformation that’s already been debunked. I recommend reading my other comment thread
yeah. when my metabolism was like this I was not sedentary but was not into sport. mosquitoes loved me (and still do) and they detect carbon dioxide so my bet is just more resting calorie burning or just an inneficient digestive system and maybe both.