i was wondering whether coinbase itself actually buys or sells bitcoin, or if they just pass along orders to exchanges or market makers that execute the trade,

i was wondering whether coinbase itself actually buys or sells bitcoin, or if they just pass along orders to exchanges or market makers that execute the trade, and then keep a record of the transactions that are executed. if coinbase fills orders by buying and selling bitcoin on its own account, then that is a very different business model from what i assumed they did