Why would one say Donde esta and not Donde es?

The difference is that in ESTAR is for location and SER is for existence (among other things, but that's getting ahead).

¿Dónde está el baño? (where is the bathroom located)

Está al fondo del restaurante. (the bathroom is located in the back of the restaurant)

Es la primera puerta a la derecha. (the bathroom exists - can be identified by - the first door on the right)

It's possible, but rare to find SER used with DONDE, but it's possible...see the contrast here. Remember ESTAR is location and SER is existence.

¿Dónde está la salida? (where is the exit located..you know there is an exit somewhere)

¿Dónde es la salida? (you're dreaming you're trapped in a maze and can't find the way out...you scream this out hoping that someone will answer that an exit exists)

Generally speaking when talking about locations, where somwething is at you use estar. Also, but not always, estar refers to things that are not permanent, although the door question is one of the exceptions.because a door is presumably permanent

Es is used, but not always when you are talking about more permanent features or characterstics

For example Ella es linda. She is beautifol. That's because beauty is assumed to be a permanent feature

Hope this helps

Suerte

NATIVE SPANISH SPEAKER

The response did not use "donde", so "donde es" did not apply here. S/He was just saying "(It) is the first door to the right."

We do the same in English.

"Where's the bathroom?"

"It's over there." (Not "Where is over there.")

I'm not sure "¿Donde es?" is ever really used..."¿Donde hay?" would be more likely if estar is not appropriate for some reason. I'm not literary-fluent, just raised in the language informally...English is what I learned academically.

Where is the bathroom? What is the location of the bathroom? ESTAR IS FOR LOCATION.

Es la primera puerta. It is the first door. THERE IS NO LOCATION HERE. It doesn't say "It is ON the first door. It says

It IS the first door.

¿Dónde ESTÁ el baño?

Where IS the bathroom?

es= is like the verb "to be"

So, we do not ask "Where is "to be" the bathroom, ¿Dónde es el baño?

but rather, "Where is the bathroom?" ¿Dónde está el baño?

El baño ESTÁ en el primer piso.

The bathroom is on the first floor.