How does a UK based ETF track a foreign market if its closed

This may be a very stupid question but I was wondering how Vanguard ETFs in the S&P 500 can track the market when the market they operate in is closed whilst the market they track is open. I don’t see my Vanfuard S&P500 ETF fluctuate past 16:30 despite the fact the market it tracks still trading.

I appreciate this might be an odd question but one I’ve wondered about a lot since becoming interested in ETFSs

Well as you said the ETF doesn’t fluctuate past 16:30 because the market is closed and so its price can’t change.

However, the value of the holdings of the ETF still fluctuate as normal while the US market is open which in turn impacts the price when the LDN market reopens.