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.
He eats less than he burns. Could be just high level of base activity - even just walking around a bunch makes a huge difference, much more than sports. Or could be very low appetite.