Learn French: Il aime voyager à vélo. - He likes travelling by bike.

In English, we can say "He likes to travel" or "He likes travelling." In French, when a verb directly follows a preference verb like aimer (to like), the second verb must always be in the infinitive (the unconjugated dictionary form). French doesn't have an "-ing" form that works the same way as English, so you simply use voyager.

Why is it à vélo instead of en vélo?

In French, the preposition used for transportation usually depends on whether you are on the vehicle or inside it. You generally use à for open vehicles you sit on (like à vélo for "by bike" or à moto for "by motorcycle"). You use en for closed vehicles you sit inside (like en voiture for "by car" or en train for "by train").

Based on this lesson's theme, how would I ask "What does he like?" using the in-situ method?

To ask a "what" question using the declarative word order (in-situ), you use the question word quoi (what) and place it where the object normally goes. So, replacing "travelling by bike" with "what", you get: Il aime quoi ? (Literally: "He likes what?").

Is vélo the only word for bicycle in French?

Vélo is the most common, everyday word for "bike". You might also come across the word bicyclette, but vélo is much more frequently used in both spoken and written French today, similar to how "bike" is used more often than "bicycle" in English.