They can loan your shares out whether they are bought with cash or margin. Some brokerages allow you to opt of loaning your shares to short sellers. Robinhood

They can loan your shares out whether they are bought with cash or margin. Some brokerages allow you to opt of loaning your shares to short sellers. Robinhood pays you part of the interest made if they lend your shares out, but as far as I know there is no way to opt-out of share lending on Robinhood.