Is there any difference between a given cream and mix of different milks resulting in the same fat percentage?
For example, is there any difference in a half-and-half cream, say 16% of fat, and a 50% mix of skim milk and 33% fat heavy cream?
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What is the application?
For most uses, no, there would be little to no effective difference.
Dairy products are a mixture of water, sugars, minerals (which the dairy industry calls ash, because they are left as ash when burned in a calorimeter), proteins, and fat.
If the product is not homogonized, the fat globules will eventually rise to the surface (where they can be skimmed as cream, giving rise to the term skim milk for when the cream has been removed). If the product is homogonized, the fat has been forced into much smaller globules which form a stable emulsion, and will not rise.
Whipping non-homogonized cream is much easier, but most of us (at least in the US) do not have easy access to such a product.
Additionally, most dairy products are pasteurized in one of the various methods, slow or fast, which can affect the flavor.
Other than these differences in flavor or whipability, you can mix them with little difference for most applications.
Note: to your specific question, your mixture of smim milk and cream may actually be richer than commercial half and half, which is often about 12% milkfat, but it would function very similarly.
Of course, all the above only applies to fresh products--basic milk or cream of various fat percentages. Fermented products like yogurt, creme fraiche, cottage cheese, buttermilk, and so forth have entirely different charactaristics.