I was slim in my 20s, I had a very active job and ate what I wanted. In reality that was croissants and pizza and Burger King! Maybe a sandwich for lunch. I don't recall being a big snacker I wasn't particularly regimented about meal times, I was hunger led. I did try and eat what I thought were grown up healthy choices like pasta or fish and veg. I had no car and walked everywhere. From this diet I did learn that too much wheat blocks my sinuses.
I went abroad and put on some weight due to environmental factors i.e. available choices.
My weight stayed like this until early 30s when I got a sedentary job then it crept up. I only really started to look at calories in mid 30s, until then I think I took pride in not having a clue.
When I did look at my diet it was more with a view to understanding the nutritional and vitamin content of food. I was trying to get away from grabbing a sandwich so I'd make my own salads etc. but in reality my intake was probably too low so I'd binge on chocolate and burgers.
This went on for ages until I finally got am exercise routine and increased my carb and protein intake so I am more satisfied and don't crave the sugar and fat. I'm slim but typically I eat three solid meals and two snacks...two eggs and toast and avocado, porridge, a large grain and protein bowl, yoghurt, fruit, nuts, olive oil, meat carb and veg for dinner would be a typical day for me, around 1800-2300 calories, I am 5.6 and in my BMI range, I don't often have food that is very high fat, refined sugar or refined carb and that just puts me on a rollercoaster of hunger, or I see immediate body fat gain (I'm 43). My body is a completely different composition now (it was 44% body fat and now it is 25% body fat). I also can't remember when I last had alcohol.
I would say - just my observation - very slim eaters sometimes get calories from alcohol or sugary treats. You can't survive on nothing. DM hardly eats much but has a can of cider every night.
I think 80-90% good is the aim in diet but many people underestimate the treats so it may be more like 60% good etc.
I don't believe you can fix diet alone without looking at daily activity levels of walking and doing some resistance training as muscle is more metabolically active and we lose muscle as we age - 0.5% a year. You don't need to physically look muscly, the back and core have lots of internal muscles that need to be strong.
Not sure if that helps but in short it was intuitive eating but more recently been environment and knowledge for me.