At the end of the day, it's all "calories in / calories out", but the real trick is figuring out how to change your calorie balance in a way that works for you, long term.
Some people do really well on some sort of rigid "I will eat an exact, precise amount of food each day", be it something like Nutrisystem or Soylent or just very carefully counting calories each day; other people can't stick with it. Some people do better just adding a solid, regular block of exercise each day, be it the gym or just walking; other people have trouble carving out the regular time. Intermittent fasting, eating windows, changing the composition of your diet, eliminating snacks or liquid calories, taking up a sport, becoming a gym rat ... there are a lot of reasonable ways to change how many calories you're consuming or how many you're burning.
The hard part -- besides actually doing it, because that's rarely easy -- is finding one that you can commit to forever, so you aren't, as you say, just trying out yo-yo diets.
Exercise is great, but you can't out-exercise a bad/high calorie diet. Running a mile on a treadmill (if I believe the computer) burns about 135 calories. A typical Big Mac meal is about 1,100 calories.
If I don't pay attention to what I eat, and am fairly inactive, I tend to eat about 50-100 calories more per day than I burn, amounting to a 5-10 lb/year gain.
That is enough of a gain that it really adds up over time, and once it has added up it is quite daunting to deal with, but as far as maintaining goes, it's very reasonable to just add a small amount of light exercise on a daily basis.
At the end of the day, it's all "calories in / calories out", but the real trick is figuring out how to change your calorie balance in a way that works for you, long term.
Some people do really well on some sort of rigid "I will eat an exact, precise amount of food each day", be it something like Nutrisystem or Soylent or just very carefully counting calories each day; other people can't stick with it. Some people do better just adding a solid, regular block of exercise each day, be it the gym or just walking; other people have trouble carving out the regular time. Intermittent fasting, eating windows, changing the composition of your diet, eliminating snacks or liquid calories, taking up a sport, becoming a gym rat ... there are a lot of reasonable ways to change how many calories you're consuming or how many you're burning.
The hard part -- besides actually doing it, because that's rarely easy -- is finding one that you can commit to forever, so you aren't, as you say, just trying out yo-yo diets.