How does a put transaction work?

Say disney is currently at $92.

The put seller (Jack) agrees to buy shares at $85 by next friday..

Today is Friday. Lets say on Monday it drops to $80.

The put buyer (Jill) exercises the option.

Who is selling 100 shares to Jack?

I was told a put buyer (in this case Jill) doeant need to own shares in order to buy puts. So if Jill doeant own shares, who is selling Jack those 100 shares when Jill exercises the put option?