If you want to learn about nutrition, you need to know enough about science to recognize when claims are being based on bad science. Science is like a god for the secular world so all these diet gurus try to convince people that their method (which they make money from) comes from science. They'll quote studies and use big sciencey words to create an impression of science when there is actually no real support from science at all. McDougall is an example of this. You need to learn how to spot these people or you will be fooled.
Even when you find nutrition information based on good science there is still a fair bit of individual variation in how people respond to foods. For example, some people cannot handle oxylates, a substance found in many vegetables. It is like a poison for them and they get sick if they follow the common advice to eat lots of vegetables. To some extent, you need to experiment and find something that works for you.
I was once really sick: rising blood pressure, prediabetic, stiff achy joints, rashes, fatigue, etc. And I seemed to be getting steadily worse. My doctor could not figure out what was wrong or how to fix it. I heard of the carnivore diet and decided to give it a try out of desperation. I ate only meat and within months had reversed all these problems. I found something that helped me but it might not help somebody else.
Even the seemingly obvious advice to eat less food won't necessarily work for everyone. Sometimes all this does is lower the metabolism while leaving people constantly hungry. They end up no longer able to resist the hunger so they resume eating normally but, because their metabolism is lower, they gain even more weight than they started with.
To find the best way to lose weight for you, try things that are based correct principles of nutrition and then see how they work for you.