Strictly speaking, there is no such thing as a countable or uncountable noun. There are nouns that are predominantly used as countable, or as uncountable, but it is almost always possible to invent a sentence in which a word takes on the opposite role to its usual one. The same is true for parts of speech such as "nouns", "adjectives", and "adverbs". Although words often belong predominantly to one of these categories, it is most often possible to imaginatively use it in other roles. So, yes, "dirt" is predominantly an uncountable noun. That is why we normally speak of "much dirt" rather than "many dirts". However, YOU ARE IN CHARGE. If you wish to think of "dirt" as something countable you are free to do so, and it makes perfectly good sense to do that if you want. For instance you might have three piles of dirt: a red pile of clay, a sandy pile, and a fertile one. In that situation, it makes perfectly good sense to say you have three dirts. You are in charge! I once tried to think of a noun that couldn't possibly be uncountable. The best example I could find was "point". I find it extraordinarily difficult to think of an uncountable way of using "point" because, as Euclid pointed out, a point has no parts.

In general, uncountable nouns can be made countable if it's clear from the context that you mean different types of the thing (like your fruit) or you mean countable units of it. There serve many different teas in that cafe. (different types of tea) I'd like a coffee and two teas, please. (one cup of coffee and two cups of tea) In your example, we can infer that it means different types of dirt. Dirt is unusual here, though, as it is usually a negative word. You would normally say ceramic is made of baked earth, rather than baked dirt. In fact you could be more specific and say clay, as this is the type of earth that is used. "Products which are made from clay(s) and are subjected to high temperatures are known as ceramics."

‘Dirts’ is correct and you’ve understood why the ‘s’ is correct here. There are several types of dirt which can be used to make things this way. Historically the word ‘ceramics’ comes from pottery, which is made of clay, one type of dirt. But the more general meaning of ‘ceramic’ is an inorganic non-metal and these have many uses beyond pottery.

Let me mention a side issue of word choice. "Dirt" is not the right choice here. "Dirt" usually carries the idea of filth, such as excrement. "Earth" would be a neutral term. However, it it is still not right, because ceramics cannot be made from just any earth. They are made from a particular kind, called "clay." "Products which are made from clay and are subjected to high temperatures..." is correct and carries the idea that ceramics are made from clay--rather than some other category of material like metal or wood or plastic. "Products which are made from clays and are subjected to high temperatures..." is also correct and carries the idea that there is more than one kind of clay, and that different ceramics might use different kinds.

In English, "dirt" is indeed an uncountable noun and does not typically have a plural form. The correct term should simply be "dirt" even when referring to different kinds or sources of dirt. Therefore, in the sentence you provided, "dirts" is a mistake. The corrected sentence should be: "Products which are made from dirt and are subjected to high temperatures are known as ceramics." Or, if you want to emphasize different types of dirt, you might say: "Products which are made from various types of dirt and are subjected to high temperatures are known as ceramics."