China owns about 7% of U.S. Treasury debt. That's the largest share owned by any foreign country, but it's hardly "all our debt."
“I have little doubt that China can ruin us economically because they own all our debt”
Out of our about $20 trillion in outstanding Treasury notes, the Federal Reserve is by far the biggest single holder - and they could buy several times more than China currently holds, should they decide to do so.
Here are the top ten foreign holders (Sep 2018, in billions):
China, Mainland 1151.4
Japan 1028.0
Brazil 317.0
Ireland 290.4
United Kingdom 276.3
Luxembourg 227.2
Switzerland 226.9
Cayman Islands 199.4
Hong Kong 192.3
Saudi Arabia 176.1
Also, China needs dollars to settle accounts at their ports for imports, so they must retain about half of their holdings, just to keep enough dollars in their “checking account” to keep goods flowing. They need foreign reserves to support (control/manipulate) their currency as well. Their position is much more tenuous than ours.
They have the means and skills to make serious trouble in our markets in the short term, but not yet enough to make permanent damage.