For most kids it's diet -- specifically, sugar -- and exercise. Not "metabolism" or GMOs or parsing insignificant differences between sugar and fructose. Diet and exercise.
Exceptions are very rare. If we're honest with ourselves and track the data a bit, we'll see why we or other family and friends are overweight, while others are thin even if they're not necessarily fit. Heck, some of my friends are overweight but physically fit. They ride bikes and exercise, but also eat a lot and drink a lot of beer, more calories than they burn up with exercise.
In the 1960s, as a kid in a big town suburb just north of NYC, I saw a few overweight kids. At the time I thought it must be genetics. Looking back... nope, it was diet and exercise. There was easier access to junk foods at the ever-present corner stores. Back then the schools didn't have vending machines filled with junk food and sodas. The cafeterias served basic nutritional meals. Heck, I often skipped the school cafeteria lunches and walked or ran to the corner store for a Hostess cherry or lemon pie and soda. Usually ran. The school bullies hung out in the playground and sidewalks to shake down the smaller kids for lunch money, so you had to run. That probably burned off my junk food calories.
Kids were free-range, hung out on the playground shooting hoops, riding bikes or just walking everywhere. Lots of activities in school including gym or sports almost every school day for at least 30 minutes. Same with the parochial school I attended for a couple of years in elementary school -- the nuns would herd us outside for structured physical exercise, then shoo us off to play.
As a teenager we moved back to Texas in the early 1970s. Big change in school lunches. It was mostly greasy carb junk -- burritos and cardboard pizzas. But no vending machines at that time. And no corner stores near our rural school. The only time sodas and candy were available around school was during sports events when the snack bar was open.
So most rural Texas kids and even most city kids were lean. Rural kids especially had almost no excess body fat. They looked like athletes, and some probably worked harder doing chores before and after school and on weekends. My sport was boxing and I thought I trained hard. But the hardest workout I ever got was loading hay with a friend on his parents' farm. No wonder he looked like an athlete, all lean, rock hard muscle. Same with the goat ropers, the rodeo kids.
There wasn't much on TV out in the country. No cable TV. Most kids were free-range and couldn't wait to get out of the house. Summers were running around, hanging out at the lake, swimming, mostly outdoor activities.
Nowadays most kids I see in Texas have at least a thin layer of fat over their entire bodies. They look soft, pasty, rarely with that muscle definition we used to see. They don't just hang out or play outside. They get little or no physical activity in school unless they pay to participate in sports -- the less well off parents can't afford pay-to-play so the poorer kids get fatter. The schools are filled with vending machines dispensing sugary drinks and snacks. TV ads bombard kids with propaganda for colorful sugary dopamine inducing drugs disguised as "food". They're overwhelmed with homework, far more than we had as kids in the 1960s-'70s. They're stressed out far more than we were, confined and herded like cattle, under the ever-present "zero-tolerance" consequences for any infraction. No wonder they indulge in comfort foods. It's self-medicating to relieve stress.
One branch of cousins in my family are chronically, morbidly obese across generations. That fed into the myth of "low metabolism." But it's just diet and exercise. I've been around them a lot. They eat lots of junk carbs, lots of sugar, and absolutely do not exercise or do any physical activities, ever. No gardening. No walking farther than from the house to the car. Nothing. After awhile diabetes and heart disease set in, they overeat even more due to being unhappy, and it rapidly gets much worse. The exceptions are the alcoholics who mostly drink and rarely eat. They're thin and remarkably healthy, relatively speaking, compared with their family members who don't drink.
I was rail thin all my life until I was in my 30s. I got busy with work, travel, and activities that seemed to keep me busy but were mostly not physically active. I developed a pot belly, which I remember some former coworkers pointing out at a convention a year or so later. It sneaked up so gradually I didn't notice. But I remember stuffing myself at lunch, especially all you can eat cafeterias, including two or three desserts. I drank more beer -- rarely more than two or three a day, but that's a lot of empty calories when you don't exercise. Over a decade from age 30 to 40 my weight went from 155 lbs to 205. And it stayed there until my early 50s when I finally changed my diet. Then in 2015 I resumed cycling. I'm back down to 150, and still have a bit of fat around the waistline, belly and lower back. But I have less muscle than when I was younger so it's distributed differently.