She talked about it in a podcast from March 2019, that it was a 'recent' thing but didn't specify. Sydney Sweeney was already listed as 'Euphoria' cast in June 2018 though.
It's certainly possible that she auditioned for it but there's also gotta be any number of random projects requiring nudity from young actresses.
She probably wouldn't have done the euphoria level of nudity you guys are expecting, she has technically been "nude" in American boogywoman and school spirits, if you can call it that no tits or ass even, forget about full frontal, pussy etc.
yes, her nipples and areola both are small and pale but a well made x-ray can make it significantly more visible than it is right now without faking anything.