«cats» in «cats meow» and «apples» in «I like apples» are not categories, they are bare plural noun phrases with generic reference. They refer generically to members of the relevant class, rather than naming the class itself.
Both are count nouns, and, in English, a plural count noun takes the null article when it is used with generic or non-specific indefinite reference rather than to denote a contextually identifiable set.
However, in
Cat is divided into numerous breeds.
Cat excludes animals such as dogs and foxes.
Apple excludes such fruits as pears and peaches.
Apple contains numerous cultivated varieties.
Both are count nouns, and, in English, a plural count noun takes the null article when it is used with generic or non-specific indefinite reference rather than to denote a contextually identifiable set.
However, in
«cat» and «apple» are categories.