Hi doma,
So increasingly, in future, some US people will owe other US people money. Net zero.
The fact the US finances its own debt is not a bad thing, per se.
The fact that foreigners are buying less US debt reflects the fact the US is running smaller trade deficits and the dollar isn't overvalued (eg relative to the renminbi).
I thought folks used to get upset that China was buying too much debt from the US. Now it is that they are not? Can't win.
Re $6tn - that's just the difference between inflows and outflows. You have to look at total expenditures to see where the money goes.