There are a few parties (called Authorized Participants) who are allowed to mint ETF shares by handing the fund the underlying, or redeem ETF shares in return f

There are a few parties (called Authorized Participants) who are allowed to mint ETF shares by handing the fund the underlying, or redeem ETF shares in return for the underlying. These APs are chosen carefully, and have the express job of creating liquidity. They make their money on arbitrage, thus keeping the ETF price closely matched to that of the underlying.