Lending liquidity comes from the public. Anyone can lend on dYdX by clicking 'deposit' the 'Account' Page.
The ETH-DAI (dYdX) market liquidity comes from our own exchange which uses cryptographically-signed off-chain messages to establish a valid orderbook. Orders can be placed by external market-makers or by users on the Cross trade mode.
The ETH-DAI (Oasis) market liquidity comes from Oasis Trade.
If you are interested in market making / providing spot liquidity, please get in touch with us!