You are talking about something entirely different.
If everyone in the US stopped paying taxes the government would not have money to spend. They couldn't just print it to pay themselves and whatever else. They couldn't even take out loans because there would be no money from us to pay for them.
So it isn't in the end government money. They use taxes collected from us as well as loans that we, ultimately, pay for, to fund the operation of government and all other programs and activities.
Without taxes government and everything funded through taxation, directly or indirectly, would collapse.
Fiat money is, at the simplest, about a dollar no-longer being backed by an equivalent amount of gold. That has nothing whatsoever to do with the use of tax revenue to fund programs. Not one thing. Any program funded through taxation is being funded by you, me and every other tax-payer in the nation. It's our money they are spending.