Learn Russian: Этот рабочий работает много.

In Russian, some nouns are actually adjectives that act like nouns. Рабочий describes someone who works, but is used as a noun to mean a male worker. Because of this, it changes its endings just like an adjective would when you use it in different grammatical cases.

Why do we use Этот instead of Эта or Это for 'this'?

Этот is the masculine form of 'this'. Even though рабочий has an unusual ending, it refers to a male worker and counts as a masculine word. Therefore, the word for 'this' must match it by taking the masculine form.

Are рабочий and работает related?

Yes, they share the same root, работ-, which is related to work or labor. Recognizing roots is a great way to grow your vocabulary. Рабочий is the worker, работает is the action he is doing, and you might also know работа, which is the noun for the job or work itself.

I learned that много means 'many' and changes the next word to the genitive case. Why doesn't that happen here?

You are right that много means 'many' or 'a lot of' when talking about quantities of things (like 'many workers'). But here, it describes the verb работает. When много describes an action, it simply means the person does it 'a lot'. Since it is not describing a noun here, there is no word to change into the genitive case.