Most people start out imitating others, mastering them and getting them set in your voice and then from there, once they have a whole repertoire of them, they can spontaneously pick and choose which ones they want to do in performance. Eventually, if they're very good, they'll develop their own.
Jennifer Holliday has talked about imitating Aretha Franklin, Patti Labelle, Gladys Knight and Barbara Streisand when she was a little girl in her bedroom listening to their records. Over the course of time (and many years soloing in the gospel church) she didn't need to imitate anyone anymore and after she became famous, people started imitating her.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney