When you short a stock, you borrow shares from someone who is holding that stock and their broker gets money for lending the shares and sometimes the holder of the shares lent out gets money. You sell the shares, probably to a market maker. The cash is credited to your account and held as collateral.

Sometime later, the stock has fallen and you decide to close the position. You buy back the shares with the borrowed money probably from a market maker and close your position. You give the shares you borrowed back to the lender. Your net profit is sell_price - buy_price - borrow_fees, anything left is your profit.

Stocks are not zero sum like options or futures, they also have no expiration date (unlike derivatives), it’s possible a short seller sold shares to someone who later profited, and then it’s also possible to buy the shares from someone who profited, even if you made a profit on shorting the stock.

So the answer is “other market participants” who also may have profited on their buy or sell.