Learn Finnish: Poika oppii, ja hän ymmärtää hyvin. - The boy learns, and he understands well.

Both are Verb Type 1 words (oppia and ymmärtää). To make the hän (he/she) form for this verb type, you take the stem and double the last vowel. The stem of oppia is oppi-, so adding another i gives us oppii. The stem of ymmärtää is ymmärtä-, so adding an ä gives us ymmärtää.

Why do both verbs keep their strong consonants (pp and rt) here, when they change for "I" and "you"?

This is consonant gradation in action! Consonants weaken (like opin or ymmärrän) when the personal ending closes the syllable, such as adding -n for "I" or -t for "you". Because the hän form just adds a vowel, the syllable stays open. Open syllables keep the strong consonants, which is why we see oppii and ymmärtää.

Is hän always used for "he" and "she"?

In standard and written Finnish, yes. Hän is strictly used for humans. However, as a quick reminder for when you listen to native speakers: in everyday spoken Finnish, people almost always use se (which literally means "it") to refer to people instead!

Why does the sentence use hyvin instead of hyvä?