This is now only of historical interest, but for a brief period between April 18, 2002 and November 30, 2003, after all Taiwan flights were moved to Narita but before shuttle flights to Seoul started, Tokyo-Haneda Airport HND
served only domestic flights. With 62,876,182 passengers in 2003, this was and is almost certainly the busiest purely domestic airport the world has ever seen. (Even after subtracting those Seoul shuttle passengers from that figure: at a peak of 8 flights/day, on narrowbody aircraft with a capacity of around 150 people, we’re looking at 40k tops for the one month they operated.)
Source: https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Japan
Since China was mentioned in the OP, I took a look at the busiest airports in China. The busiest one without “International” in its name appears to be Zhuhai, with 11.2M passengers passing through in 2018. This makes it the 35th busiest airport in China.
I also looked at airports higher up on the list. Airports #21–34 all appear to have at least one international flight according to flightconnections.com. I didn’t bother looking at the 20 most busy airports, since it seemed obvious to me that they were major cities with international connections; but it’s possible I missed one.
According to Wikipedia, São Paulo–Congonhas Airport served over 21 million passengers in 2017. It used to be international, but
Since the opening of Guarulhos Airport in 1985, international flights no longer operate from Congonhas (…) In 2008 Congonhas lost its international category.
Credit:stackoverflow.com‘