https://www.investor.gov/introduction-investing/investing-basics/glossary/exchange-traded-fund-etf#:~:text=Exchange-traded funds (ETFs),by SEC-registered investment advisers. ETFs are private companies, they can run their private market as long as they want. An usual way of working is to track the underlying asset during trading hours and the futures markets after hours.
I just posted (yesterday) a chart with after hours (upto 8pm restarting 4am). No need for mysterious private market stuff.
No, you don't understand. What TS intends to ask is: (why) do these ETFs trade until 16:15 in REGULAR TRADING (the title of this topic is confusing in relation to the actual (not good phrased) question of the TS...)
OK but in the body he asks very specifically "why they continue to trade after the market close for an additional 15 mins." So that's probably his real question (title is just a generic phrase). Point is, it could be that most people don't know that some stocks have extended regular trading (i.e. with the RT rules, not the PM/AH rules). If that's the case than the question is not understood anyway. Therefore my remarks...