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Massive explosion rocks SpaceX Texas facility, Starship engine in flames
(interestingengineering.com)
This is a most excellent place for technology news and articles.
That's actually a really good question. The short answer is that we don't remember how to. A lot of the techniques used to actually make the parts were poorly documented. That was partly on purpose, everything was top secret because we didn't want the Russians to know how we were doing it all. And now, all the people who did those jobs have gotten old and left the industry.