Learn Spanish (Spain): Se aprende mucho viajando.

The word se creates an impersonal sentence, which means it refers to people in general, translating closely to 'one learns'. While you can sometimes use the informal you (aprendes) to make general statements, using se with the third-person singular verb is the most natural and common way in Spanish to state general facts.

Where is the Spanish word for 'by' in 'by traveling'?

In Spanish, you do not need a preposition to say 'by doing something'. The gerund form alone, which ends in -ando or -iendo, expresses the method or how something is done. Therefore, viajando all on its own directly translates to 'by traveling', and adding a word like por before it would be incorrect.

Why does mucho not change its ending to match a noun here?

Because mucho is acting as an adverb describing the verb aprende, telling us how much is learned. When mucho modifies a verb rather than a noun, it does not change its ending to match gender or number, so it always stays in its default masculine singular form.