Learn Russian: Это удивительный фильм.

Here, Это acts as a standalone pronoun meaning 'This is'. When used this way, it always stays in the neuter form Это, regardless of the noun's gender. If you wanted to say 'This film is amazing' instead, you would use it as a pointing word: Этот фильм удивительный.

How would I change this sentence to say 'This is the most amazing film' using the grammar from our current lesson?

You would add the masculine form of самый (the most) right before the adjective. The sentence becomes Это самый удивительный фильм. Both самый and удивительный must match the masculine noun фильм.

Why does удивительный end in -ый rather than -ий?

The adjective must match the masculine, singular, nominative noun фильм. The standard ending for masculine adjectives is -ый. We only use -ий if the stem ends in specific letters like г, к, х or the hissing sounds ж, ш, щ, ч due to Russian spelling rules. Since the stem here ends in н, it simply takes the regular -ый.