China publicly acknowledged 5000 km of the "Underground Great Wall" which was "completed" (likely still on going) more than a decade ago. Georgetown University did an analysis on the program and concluded China had 3000 nuclear warheads due to the scale of the tunnel networks which is likely to be an exaggeration. China maintains a minimum deterrence and the length of the tunnels are needed to cover its large geography, be robust against a strike and be unpredictable in missile launch location.
The tunnel network includes roadway system, rail system and support facilities. During drills, members of the "second artillery" now called "rocket force" live in underground facilities for many weeks at a time.
The underground facilities are occasionally referenced in media. The movie "Sky Hunter" for example was produced in coordination with the Chinese military and were allowed to use military facilities and equipment during filming. During interviews it was said by the PLA official that "we didn't show everything but what we did show reflects reality". This was in reference to the equipment and tactics, thought it's more of a equipment showcase.
@22:52 in the Sky Hunter movie, it showed what looks to be an old declassified underground facility built many decades ago (60s?). It is likely unnetworked with other tunnels,a stand alone facility like the "816 project" which is open for tourism.
Full Sky Hunter movie
Link to Underground Great Wall thread and the Georgetown report:
https://defence.pk/pdf/threads/pla-underground-great-wall-analysis.509270/