Did you know that dogs have essentially two "food-related" senses? Obviously taste, which contrary to what you might think isn't the most important, and smell.
Dogs have around 1700 taste buds, certainly more than cats, which have about 500, but way less than the 9000 in humans.
This explains, for example, the poor tendency of our dogs to "savor" the food we offer them and prefer to swallow it quickly, almost without chewing.
Even though they have few receptors available, they can perceive different flavor nuances quite accurately.
This allows dogs to develop important subjective preferences, which often cause some owners frustration, as they no longer know how to feed them because they're "spoiled" or simply have a picky palate!
They are able to clearly distinguish (and prefer!) sweet flavor, unlike cats which almost have no specific receptors for it.
However, these foods aren't suitable for their metabolism due to the excess sugar present,
so they should be avoided as much as possible or at least limited to the fructose found in some fruits, which they can safely eat.
Besides that, they can distinguish between salty, umami — that is, the flavor of the nucleotides that make up protein sources like meat and offal, which they enjoy so much — while sour and bitter aren't very appreciated. It goes without saying that this leads to a preference for meat-based foods or their substitutes.
However, food selection mainly happens through the sense of smell, which is much more developed and evolved than our sense of smell, unlike taste: while humans have 10 million olfactory receptors, dogs have around 150-200 million!
This is exactly what we can work on to solve those slightly annoying problems with appetite and interest in food.
In this regard, it's important to remember that foods release aromas of varying intensity depending on the temperature they're served at and also the amount of water they contain: a wet food served at room temperature or just slightly warm will be much more appealing than a dry or dehydrated one, like kibble.
The most attractive smells, of course, are those that come from protein sources like meat or offal (liver, heart...) and fats, always of animal origin (it's no coincidence that they're carnivores!)
If your dog also has problems with loss of appetite, try varying their food based on the smell and the temperature it's served at.