Learn Russian: Он зарабатывает мало.

The infinitive is зарабатывать (to earn). You might recognize the core part from работать (to work). Adding the prefix за- changes the meaning from just working to actually getting paid for it.

How does мало connect to this lesson's rule about the genitive plural?

In this specific sentence, мало acts as a simple adverb modifying the verb. But if you add a noun, words of quantity like мало (little/few) and много (a lot/many) trigger the exact same rule as numbers 5 and up! For example, to say 'He earns little money', you must put money (деньги) in the genitive plural: Он зарабатывает мало денег.

Could I change the word order to Он мало зарабатывает?

Yes, Russian word order is very flexible! Both versions are completely natural. Putting мало at the very end just gives it a bit more emphasis, highlighting the fact that the amount is small.

If I want to say the opposite, what word replaces мало?

You would use много, which means a lot or much. To say 'He earns a lot', the sentence would simply be Он зарабатывает много.