Alice holds SpaceX stock and believes it will rise. Bob believes the stock will fall. Alice and Bob reach an agreement for Alice to "lend" their SpaceX stock to Bob for a small "fee". Bob immediately sells the SpaceX stock at the current market value. After some time Bob will buy back the sold SpaceX stock at the current market value (hopefully less than Bob sold it for) and return the "borrowed" SpaceX stock to Alice thereby fulfilling the original contract.
It's also possible Bob's thesis on SpaceX could have been wrong and the shares could skyrocket. There's usually a provision in the contract for Alice to recall the shares she lent to Bob. In this case, Bob would be forced to buy SpaceX stock at the current market value and likely lose money on the overall trade.
To answer your specific question, "Who do you make money from?" It's actually not clear. Bob selling-high and buying-low doesn't necessarily mean whom Bob sells-to and whom he buys-from are on losing sides of the trade despite Bob making a profit. E.g. the buyer of Bob's short-sell could write calls and the stock could close pass the strike on expiration and turn a small profit as well.
It's also possible Bob's thesis on SpaceX could have been wrong and the shares could skyrocket. There's usually a provision in the contract for Alice to recall the shares she lent to Bob. In this case, Bob would be forced to buy SpaceX stock at the current market value and likely lose money on the overall trade.
To answer your specific question, "Who do you make money from?" It's actually not clear. Bob selling-high and buying-low doesn't necessarily mean whom Bob sells-to and whom he buys-from are on losing sides of the trade despite Bob making a profit. E.g. the buyer of Bob's short-sell could write calls and the stock could close pass the strike on expiration and turn a small profit as well.