In short, experience. Veteran and naturally gifted musicians have gained, mostly via trial and error, an understanding of musical structure and composition that the casual listener lacks.
Basically, after enough practice, you know that in a given format, the next logical step for “Musician A” will likely be “X”; thus allowing you to plan what you will play next in order to compliment it.
There’s probably a much simpler way to describe all of this but hope this helps
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