That 'borrowed' position has to unwind sometime in the future and to unwind it requires a share purchase. Now if no one is selling where do the shares required

That 'borrowed' position has to unwind sometime in the future and to unwind it requires a share purchase. Now if no one is selling where do the shares required to fulfill these burrowed position come from? If it was not that buy pressure created by the excessive short selling, what did drive up the share price?