Fiscal policy is the impact of government actions on the economy. Monetary policy is the impact of central bank actions on the economy.
The situation the USA now finds itself in is being called Fiscal Dominance. The idea is that when not in fiscal dominance the central bank can adjust the money supply by controlling how much money it lends to banks which then lend into the economy. So adjusting the interest rate the central bank lends at can control the money supply and the inflation rate. But in fiscal dominance the main factor is the central government's huge debt and deficit levels which cause them to sell lots of bonds and spend lots of money into the economy. The central bank raising or lowering the interest rate will not really impact the rate at which the central government spends money. In fact, higher interest rates will increase the governments interest expense and make them spend even more money. The government will lean on the central bank to keep a lid on interest rates to keep their interest expenses lower. So in this "fiscal dominance" situation the central bank can no longer control inflation by raising interest rates.
Lyn Alden has popularized the term Fiscal Dominance and also the phrase nothing stops this train. She points out that the only real cure is for the government to drastically cut spending and that there is no real chance of that happening, so nothing stops this train. She wrote a book Broken Money that covers this.
I still think the death spiral view is the best way to look at it. The government needs to sell bonds to get cash to do all the deficit spending it does. If the world does not want to buy as many bonds as the government is selling the central bank will be forced to print money and buy bonds. However, as they do this they make the bonds even less attractive to investors because it will make more inflation. So more people run away from bonds. This causes the central bank to print faster, and people to flee bonds faster. You get a death spiral and out of control inflation.
Trump's trade bullying makes the world less inclined to hold US bonds. I think this brings the death spiral closer.
