Flavor – Homemade plain yogurt

The answer is that they are not much different.

During fermentation, some of the milk's lactose is converted by bacteria into protein and other substances. The amount of conversion is pretty insignificant in terms of calories and grams of macronutrients. The most important effects are the changes in taste and consistency and the presence of live bacteria. Usually many of the bacteria survive the highly acid stomach fluids to reach the small intestine where they continue digesting lactose.

Fitday.com gives the info below for a cup of each, which may be surprising at first. However, commercial yogurt tends to be more dense than milk, as you might see if you make it at home: a cup of milk gives you some watery fluid floating on top of less than a cup of yogurt.

Food              | Calories | Fat  | Carbohydrates | Protein
------------------+----------+------+---------------+--------
Whole Milk Yogurt | 149 kcal | 8.0g | 11.4g         | 8.5g
Whole Milk        | 146 kcal | 7.9g | 11.0g         | 7.9g

Overly sour yogurt is a sign of inconsistent inoculation

Check the type of culture you are using, and ensure you are keeping the yogurt above 37C for more than six hours. The yogurt should finish as a solid lump that resists pouring, not a thick liquid, or a lumpy paste

Pouring off the whey and straining the yogurt makes it more creamy and taste sweeter

Thickeners are not required, just complete inoculation

For sweet yogurt dishes I mostly use sour berries (blackberry, raspberry etc) so it is expected to be somewhat sour anyway