Look at the agreement you sign when opening a brokerage account. You give the brokerage firm the right to loan out any shares of stock you own. I believe this only happens in accounts that have margin (you do have to have a margin account to short), but I am not entirely certain.

generally from your broker (who borrows them from their other clients who own those shares). there's some institutional lending also

Apologies for my complete ignorance -- but doesnt that sound super shady?