Learn Portuguese (Portugal): Aquela médica é muito trabalhadora.

Because here muito means "very" and describes how hardworking she is. When muito means "very", it acts as an adverb and never changes its ending. It only changes to muita or muitos when it means "a lot of" or "many" and describes a noun.

What is the masculine form of the adjective trabalhadora?

The masculine form is trabalhador. In Portuguese, adjectives that end in -dor usually just add an -a to the end to form the feminine version.

Why does the sentence use aquela for "that" instead of essa?

Aquela is a demonstrative word that means "that over there". You use it when the person or thing you are talking about is physically far away from both you and the person listening to you. If the doctor were standing closer to the listener, you would use essa instead.

Why do we use é instead of está here?

We use é, which comes from the verb ser, because being hardworking is a lasting personality trait. You would use a form of estar (like está) to describe temporary states, conditions, or moods.