Learn Spanish (Latin America): Mi amigo es mexicano.

In Spanish, nationalities and languages are not capitalized unless they are the first word of a sentence. So, we write mexicano, not Mexicano.

Why do we use es here instead of está?

The word es comes from the verb ser, which is used for core identity traits like nationality, profession, or origin. Because being Mexican is an essential fact about the friend, we use es.

If my friend is a woman, how does the sentence change?

You need to change the noun and the adjective to their feminine forms. The sentence becomes Mi amiga es mexicana. The word Mi stays the same because it does not change for gender.

Does Mi change if I am talking about multiple friends?

Yes! If you want to say 'My friends are Mexican', you have to make everything plural. The sentence becomes Mis amigos son mexicanos.