Learn Spanish (Latin America): El doctor es joven y feliz.

No, they stay exactly the same! While many Spanish adjectives end in -o for masculine and -a for feminine, adjectives that end in a consonant (like the n in joven and the z in feliz) or the letter e are gender-neutral. They don't change to match masculine or feminine nouns.

Is there a feminine word for el doctor?

Yes, the feminine form is la doctora. Because joven and feliz are gender-neutral, the sentence for a female doctor is simply: La doctora es joven y feliz.

How would I make joven and feliz plural if there are multiple doctors?

To make an adjective ending in a consonant plural, you add -es. So, joven becomes jóvenes (the accent mark gets added to keep the stress on the same syllable). For words ending in z like feliz, the z changes to a c before adding -es, making it felices. The full plural sentence is: Los doctores son jóvenes y felices.

I learned that feelings use estar. Why does this sentence use es (from ser) with ?