Lending liquidity comes from the public. Anyone can lend on dYdX by clicking 'deposit' the 'Account' Page.
The liquidity for all the (dYdX) markets comes from our own exchange which uses cryptographically-signed off-chain messages to establish a valid orderbook. Orders can be placed by market-makers or by users on the Trade tab on the left side-bar of the Trade page.
If you are interested in market making / providing spot liquidity, please get in touch with us!