Learn German: Man kommt sehr schnell hierher.

The word hier describes a fixed location, like saying "I am here." But this sentence is about movement towards a place. By adding her (which means movement towards the speaker), we get hierher, meaning "to here."

Could I use hierhin instead of hierher here?

No, because hin means movement away from the speaker. Since the verb is kommt (comes), the subject is moving towards where the speaker is. That is exactly what her expresses, making hierher the right choice.

Why use Man instead of Du if the English translation often uses "You get here very fast"?

The pronoun man is used for general statements applying to anyone, just like "one" or the general "you" in English. If you used Du, it would sound like you are talking to a specific person rather than stating a general fact about how fast it is to travel to this location.

Why is kommt translated as "gets" instead of "comes"?

The literal translation is indeed "One comes here very fast." In English, we often use the verb "get" to talk about traveling to or arriving at a destination. In German, kommen is the most natural way to express this idea.