The price of an ETF is not automatically linked to the price of the underlying stocks, so it is possible for it to get slightly out of step depending on supply and demand.
However, every ETF has a set Authorized Participants associated with the ETF. At any time, they are entitled to either deliver the underlying stocks that make up the ETF to the ETF manager and receive units of the ETF in exchange, or vice-versa: exchange units of the ETF for the underlying stocks.
So suppose the "bid" price of the ETF rises above the "ask" price of the underlying stocks. An authorized participant can buy the stocks, deliver them to the ETF manager in exchange for units of the ETF, and sell those on the market for a profit. As long as the difference in prices is enough to cover any transaction costs, they have an incentive to do it. And the effect will be to increase supply of the ETF and increase demand for the underlying stocks: so all else being equal, the ETF price will drop and the underlying shares price will rise, reducing the discrepancy. The same effect applies in the opposite direction if the price of the ETF drops too far.
So the prices won't drift very far apart - roughly speaking, the price of the ETF will need to stay within the bid/ask spread of the underlying stocks. If the underlying stocks are illiquid that could be quite a wide window, but that's to be expected.
Note that depending on the sizes of the respective markets, the correction might be either to the price of the ETF, or to the price of the underlying stocks, or to both.