X is what Google used to be.
Outside of X, Google was never known for crazy moonshots. Google was always an ad platform with a search engine.
Google was the name of the search engine and the company. Now the company name has changed, everything is restructured and Google is an ad platform with a search engine.
"X Development LLC. is an American semi-secret research and development facility and organization founded by Google in January 2010, which now operates as a subsidiary of Alphabet Inc. X has its headquarters about a mile and a half from Alphabet's corporate headquarters, the Googleplex, in Mountain View, California."[0]
I think they've founded X because Google's interesting ideas got too big for a single department, so they spun it off.
We don't know how many smaller X inventions get into Google ecosystem. I'm sure that some of that research got integrated back into their data centers or networking hardware, etc.