They have to be bought - as in you have to pay for them, doing otherwise would be fraud.In shorting the shares are not bought, they're rented from someone else. Which is, frankly, ridiculous.
Source: investor.govA "short" position is generally the sale of a stock you do not own. Investors who sell short believe the price of the stock will decrease in value. If the price drops, you can buy the stock at the lower price and make a profit. If the price of the stock rises and you buy it back later at the higher price, you will incur a loss.
Short selling is just acting as a middleman between somebody who has shares they want to sell and someone else who wants to buy them.