Generally it's from brokers who are holding it on account, and they agree to lend it for a fee and some collateral.

There may be problems finding someone to lend thinly traded stock. Or you can take out a short position and not be able to buy it back at sensible prices: this results in a "short squeeze". https://www.reuters.com/article/us-volkswagen/short-sellers-...

It is possible to lose more money than you invest with short selling, so be careful.

In addition to the sibling answers about collecting interest and brokers, lending stock to short sellers is a good thing if you are a long-term investor. Shorts put some downward pressure on the price and give you an opportunity to buy more at a reduced cost. Additionally, if your thesis on the company being good is true, those shorts are eventually going to be buying back the stock after it has appreciated, ultimately creating even more upward price pressure in the long-term.

Usually people don't go to an exchange directly to buy stocks, they go to a broker-dealer: that's who lends the stock out to short.

Fees to short certain stocks can be quite high, so even if the stock dips it's possible to lose money.

Fun fact: a lot of ETFs and index funds generate revenue on the backend by lending out the stocks they hold and charging a bit of interest to the borrower.

Forgive the oversimplification, but you borrow from someone who owns the stock. They lend it because they collect fees/interest when you borrow it.

You pay the person you borrow the stock from a fee.

I believe you pay them some small sum for it, so you could call it ‘renting’ the stocks for a short while.