In the cultivation cave dwelling of Jia Chunzhen, Yu Qingtan of course didn't actually attempt to break through to the Golden Core Stage.
Her primary purpose for infiltrating the Star Moon Sect was the Moonlight Divine Water and Starlight Divine Water.
Within the Star Moon Sect, there is a secret realm of the sun, moon, and stars, and it is said that the Sunlight Divine Water, Moonlight Divine Water, and Starlight Divine Water are all produced in this secret realm.
The condition to enter this secret realm is to be a Golden Core true disciple of the Star Moon Sect, and to have at least comprehended one of the Way of the Sun, Way of the Moon, Way of the Stars, or Path of Water.