The celebrity doesn't need to know anything about perfume, and I don't think anyone necessarily thinks they wear their own stuff themselves.
The celebrity name is just being used as shorthand to help you narrow down what to look at based on the celebrity's vibe.
Britney or other pop stars are going to be playful, fruity, happy smelling scents. JLo's vibe is more sexy, sultry and grown up but not super high end. Billie's is probably darker, young but more unusual scents. Sarah Jessica Parker is more high end, expensive scents (within the confines of still being celeb perfume not artisan).