Just to clarify, many portfolios are created in a way that lending allowed is ON by default, as such people may not be explicitly by the actual owner, but something that the given trading platform does using your underlying assets.
This is similar to how the bank may lend out your current balance and not actually hold ALL of the money that its customers has in their accounts.
This is one of the things going on with Robinhood right now. If you place a limit order, they cannot lend you share out from under you. WSB people are putting limit sells at very high numbers trying to keep scarcity but RH is cancelling those limit orders.
This is similar to how the bank may lend out your current balance and not actually hold ALL of the money that its customers has in their accounts.