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.
Sometimes an actual medical condition makes the difference.
At a yearly physical, my doctor noted, “no one could gain that much weight that fast without something wrong”. That’s how I got tested for and diagnosed with a thyroid disorder. It’s also the explanation for why I could never lose weight. I thought the reason I could hardly move was because I was just getting older.
For years I had been unable to lose weight by dieting. After getting my thyroid levels corrected with medication, I became more active. I started casually intermittent fasting, and I lost 30 pounds.
There’s also been research that shows your gut bacteria has a lot to do with whether you gain weight or not.
https://www.uclahealth.org/news/article/gut-microbiome-makeup-can-determine-ability-to-lose-weight