How does an S&P 500 ETF from another market follow the S&P 500 when the US stock market is closed?

So I’m invested in an S&P 500 ETF that’s traded on the Euronext Amsterdam market (CSPX.AS).
When trading hours start here, the US stock market is closed and only opens 6.5 hours later.
How is this ETF able to trade (and fluctuate in price) when all the underlying companies are “closed” and can’t be bought?

What happens when the Euronext Amsterdam market closes but the US stock market is still open for several hours?