Is is possible to say 'aux restaurant'
"Aux" is the contraction of "à" + "les", so it precedes a plural noun.
Before a singular masculine noun, "à" + "le" contracts to "au".
Why not 'au restaurant?'
As they are working inside a restaurant the correct preposition here is "dans".
Since "au" is the contraction for "à+le", the use of "au" makes the restaurant specific, as in "the women are working in the restaurant".
Why not 'resto'? Is it slang?