There was a recent article studying children in the Amazon which concluded diet is much more important than exercise for keeping low weight:
https://www.nytimes.com/2021/02/24/w...in-amazon.html
If you read the article you can get an idea of what kind of (permanent) diet kept weight down: no processed sugars or carbs (that means no white rice or non-whole-grain flour), limited meat and dairy.
I bumped up my exercise big time at the start of the pandemic. I lost 10 lbs at first, but gained it all back as my body caught up and decided I needed to eat more.