Learn German: Braucht das Auto viel Benzin? - Does the car need a lot of gas?

Because it is a yes/no question. In German, questions that can be answered with yes or no start directly with the verb. You do not need a helper verb like "does" as you do in English; you simply move the main German verb to the very front of the sentence.

Why do we use viel here instead of viele?

Viel means "much" and is used for uncountable things, like gas, water, or time. Viele means "many" and is used for plural things you can count, like cars or apples. Since gas is a liquid substance that you cannot count without adding a measurement like liters, we use the uncountable viel.

How would I answer this question using weil to practice this lesson's grammar?

If you wanted to answer "Yes, because it is big," you would say: Ja, weil es groß ist. Remember that weil (because) is a subordinating conjunction. This means it kicks the conjugated verb (ist) all the way to the very end of the clause.

Why isn't there an article like das before Benzin?

Benzin is a mass noun here, meaning it refers to a general substance. Just like in English, when you talk about an unspecified quantity of a substance (like "a lot of gas" instead of "the gas"), you drop the article.