Learn Spanish (Spain): Viajan muy bien.

Spanish often drops subject pronouns like ellos (they) because the ending of the verb already tells us who is acting. The -an ending on viajan tells us the subject is "they" (or the formal "you all," ustedes). You only need to add ellos if it isn't clear from context.

Since we are learning question words, how would I ask "How do they travel?" to get this sentence as an answer?

You would use the question word cómo (how). The question would be ¿Cómo viajan?. Remember that all question words in Spanish carry a written accent mark!

Why do we use muy here instead of mucho?

Muy means "very" and is used right before describing words (adjectives or adverbs) like bien (well). Mucho means "a lot" or "much" and is used with nouns or after verbs. If you wanted to say "They travel a lot," you would say Viajan mucho.

Why is it bien and not bueno?

Bien means "well" and is used to describe how an action is done (in this case, how they travel). Bueno means "good" and is used to describe things or people (nouns). Because we are describing the verb viajan, we need to use bien.