Learn English – I like dog or I like dogs which is correct and why

The use of any plural noun as an adjective, adverb, adjective clause, or adverbial clause does not affect the count of the verb. The verb only cares about the subject of the sentence.

What is the subject of this particular sentence? If it were "cats and dogs", then you would be correct to believe the sentence would be "there are cats and dogs" or something similar. However, the actual subject of the sentence is the expletive pronoun "it" (sometimes, and with great protest from "it", referred to as a "dummy" pronoun).

It is unfortunate.

In this case, "it" is always a singular subject for the purposes of the verb. There is no actual subject matter with an expletive pronoun. A subject can be devised and a sentence reworded, but

it is not necessary to do so...

Yes, you can say “I like apple” if you are talking about the fruit pulp, its texture or its taste; i.e. when it is uncountable. The same is true for any fruit,

• The median proportions of food types selected are shown in Figure 1 where it can be seen that both species [rats] tended to prefer banana and avocado to the other foods offered. (source)

• Infants can be fed puréed apple after five months of age.

And when we talk about dishes, in this case dessert, the singular is used.

• ‘Twin Peaks’ the groundbreaking drama from David Lynch starring Kyle MacLachlan as FBI Special Agent Dale Cooper, the detective with a predilection for cherry pie, and a nose for mystery,... (source)

But normally when we use the word apple, we think of it as something that is countable, that we can pick up and bite into. We are talking about apples in general, not about a single apple. Which is why the phrase I like apples will always be far more common than I like apple.