Learn Finnish: Jos sataa paljon, puusto kasvaa hyvin. - If it rains a lot, the stand of trees grows well.

It is formed by adding the collective suffix -sto to the noun puu (tree). This suffix turns a basic noun into a collection or group of that thing, so puusto means a stand of trees or the timber in a specific area.

Could I just use metsä (forest) here instead of puusto?

You could, but the meaning shifts slightly. Metsä refers to the forest as a whole location or ecosystem, whereas puusto specifically emphasizes the trees themselves as a collective group. It is very common in forestry when talking about tree growth.

Why doesn't the sentence say se sataa for "it rains"?

Unlike English, Finnish does not use a dummy pronoun like "it" for weather verbs. The verb sataa (to rain) simply stands alone in the third-person singular form.

Is hyvin the standard way to turn the adjective hyvä into an adverb?

Yes! Just like "good" becomes "well" in English, the Finnish adjective hyvä (good) has a slightly irregular adverb form, which is hyvin (well).