1. Introduction: Who cooks, and who eats what's cooked?
Think of your kitchen at home. Someone lights the chulha and cooks the food. Everyone else at the table eats what was cooked. Nobody at the table made the rice from sunlight — they just ate it.
The living world splits in exactly the same way. Some organisms make their own food. Everyone else eats food that was already made. That single split is what "autotrophic vs heterotrophic nutrition" is about.
First, one word. Nutrition is the way a living body takes in food and uses it for energy, growth, and repair. Every living thing does it. The question is how it gets that food — and there are only two answers.