One of them is plagiarizing a lot of other languages.
Massive is an adjective to modify stealing, and stealing here is the noun "stealing"
Adv can be added to is, and adverbs can modify adjectives and verbs:
For example, 1. He's here. He left.
2. you are very good, here very is an adverb,
You can't modify nouns. But some words, adj and adv, have the same form and are special. This special word can modify nouns.
An express train goes very fast. If you work hard, you will work hard.
However, the adverb here typically does not modify is, but modifies the gerund stealing behind it, because: "Gerund is changed from verb and can be used as subject, object and predicative like noun. At the same time, it also has the function of a verb and can be modified by an object or an adverb, which is another feature that distinguishes it from ordinary nouns. "
The history of our English language is a history of plagiarism from other languages.
Wish progress.