Learn Spanish (Latin America): Esa parte de la película es buena.

Even though it ends in -e, the word parte is feminine (la parte). Because it is feminine, the words pointing to or describing it must also be feminine: esa parte (that part) and es buena (is good).

Since this lesson is about comparatives, how would I say 'That part of the movie is BETTER'?

You would replace buena with mejor. Just like in English, we don't say 'more good' (más buena); we use an irregular word. The sentence becomes Esa parte de la película es mejor. Note that mejor doesn't change for masculine or feminine, only for plural (mejores).

Why do we use es instead of está here?

We use es (from the verb ser) because we are describing a core characteristic of the movie. Even though you are stating an opinion, whether a movie is 'good' or 'bad' is treated as an essential trait, not a temporary condition or feeling.

What is the difference between buena and bien? Could I say es bien?