This is a very good question. We had some discussion about this in another thread and I believe some of the other debt clock posts have as well.

Here is just a bit of info I saved (C&P):

Great digging posted by u/Kingslea re US Debt Clock ownership and data gathering. Hopefully someone can verify/add to this information.

Post #1: https://greatawakening.win/p/17rSo1OOod/x/c/4Tz0NMXqF6G

The National Debt Clock is owned and maintained by the Durst Organization, which was founded by Seymour Durst, a New York real estate magnate who adjusted the tally via modem until his death in May 1995. After his death, his son Douglas became president of the Durst Organization, which owns and maintains the clock. Artkraft Strauss has been keeping the figures current since then.

Post #2: https://greatawakening.win/p/17rSo1OOod/x/c/4Tz0NMXqWHf

This is interesting: During World War II, Artkraft supplied custom metal work to the United States military, including hand-made airplane wings and containers for installation in aircraft and tanks. When blackouts and energy shortages limited their ability to illuminate their signs through electricity and neon, the company developed alternative methods, including the use of rechargeable battery packs and dangling mirrors that picked up ambient light.