Does anybody remember being taught in school that the USA was a protectionist nation until 1913? Does anybody remember learning that import tariffs were the main source of revenues for the federal government until then?

I am not an economist. If the USA runs up debt serviced by bonds, at present China buys the debt as bonds, gilts etc. This sounds ominous. This encourages US debt and keeps bond yields low, as they can easily find a buyer at present ? Surely it can all go horribly wrong if China, amongst others, stop buying ? My real question is why do they buy up US debt? What is the advantage for China in terms of dollar yuan exchange rates ?

China lowers the international value of its currency so that its exports are super-cheap! The USA used to have currency backed by gold (see the Bretton Woods Agreements) but now market fundamentals MAY finally be catching up with the USA. (Maybe. Maybe not.) Exporting more than we import would certainly help the economy in the long term.

These countries produce goods much much cheaper than in USA.
Will it be possible to keep paying high us $ salaries to workers in USA compared to other developing countries ???