When "cabbage" is used as a countable noun, we can use "cabbage" to represent many cabbages. For example, "I bought three cabbages at the grocery store." I bought three cabbages at the grocery store. She planted five cabbages in the garden. She planted five cabbages in the garden. )
But when "Chinese cabbage" is used as an uncountable noun, it means a general concept and refers to the whole Chinese cabbage. In this case, we don't use the plural form. For example, "I like cabbage in salad." I like cabbage in salad. Cabbage is a good source of vitamins. Cabbage is a good source of vitamins. )
It should be noted that "Chinese cabbage" can also be used as an adjective to modify other nouns in a specific context. Such as "cabbage patch" or "cabbage soup".
To sum up, "Chinese cabbage" is an English noun, which means cabbage. "Chinese cabbage" can be a countable noun or an uncountable noun according to different contexts and usages. When representing multiple cabbages, we use the plural form of "cabbage"; When referring to whole cabbage, we use the uncountable noun "cabbage".