According to goldsoundz, Britney recorded the complete main vocal, but never got around to doing the ad-libs so those are the writer. I'm not sure if this includes the "heeeeey yeahs" at the end because those are perfect.
Y'all, she recorded this shortly after Pretty Girls in 2015. She could've recorded the rest of the album after and paid dust to Mood Ring. Let's not turn this into another "how could her team???" despite me being an active member of that community.