"Excuse me, is the bathroom downstairs?"
Translation:Disculpe, ¿el baño está abajo?
I looked up the difference between abajo and debajo, and found a good answer here.
"Debajo" requires a reference point below which something is placed, whereas "abajo" can be anywhere lower than your reference point.
"Vamos abajo" means "We're going down" [downstairs, perhaps?], but "Vamos debajo" would make you wonder: "Under... what?" (debajo de... ¿qué?)
So basically, because we're just saying the bathroom is downstairs, we use abajo; debajo would be used if we were saying it was "underneath [something]".
It is extremely common for Spanish speakers to invert the words in a question, the same as in English. It is never wrong to invert them, and Duolingo is wrong when they reject answers that are given in in the inverted order. I have spoken Spanish fluently for thirty years and I am quite aware that native Spanish speakers are far more likely to invert the questions than to phrase them in the declarative manner that Duolingo teaches is correct.
The está in this sentence is a conjugation of the verb estar, meaning "is".
E.g. "Where is the bathroom?" - "The bathroom is downstairs." - "Is the bathroom downstairs?"
"¿Donde está el baño?" - "El baño está abajo." - "¿El baño está abajo?"
It's not esta, the feminine version of este, meaning "this".