Keep it Simple
When it comes to weight loss it's pretty simple (mechanically speaking of course)...It’s all about the deficit. Whether you’re keto, vegan, pescatarian, or you survive on pizza and ice cream alone, it's about a calorie deficit. The difference becomes how you lose weight and where you lose weight from.
If your body burns 2000 calories a day and you eat 1800 calories of nothing but pizza and ice cream (for retrospective that's about 3 slices of cheese pizza and about 2 cups of plain vanilla ice cream a day). You’ll lose weight, yupp you sure will, you’ll probably FEEL awful. And you’re more likely to lose muscle and other lean tissues than actual fat. But you’ll lose weight.
If you want to lose weight/fat and keep the strength and muscle you have gained that's where your proteins, veggies, and healthy fats come into the equation. Using the same example above you’re giving your body better quality products so it doesn't have to pull from itself in order to fill in the gaps.
Sorry to say, but it really is that simple, when it comes down to it it's all energy. “Calories=energy”, fat is made up for calories so “fat=stored energy”. So the equation is actually pretty simple, more energy going out that coming in, leads to a decrease of the energy stored.