Could be anyone. What matters is whether they're covered calls or not. I don't know if that info is available. If it's a covered call the seller of the option h

Could be anyone. What matters is whether they're covered calls or not. I don't know if that info is available. If it's a covered call the seller of the option has the shares, so they essentially sell them to whoever bought the option and decided to exercise the option. If they're not covered, then they have to go out and buy them.