Well in a nutshell, the US government makes the money. It can never run out. So it isn't dependent on tax revenue to spend.

So the limitations on spending are elsewhere.

This is a good intro:

http://elliswinningham.net/index.php/the-basics/us-currency-... http://elliswinningham.net/index.php/the-basics/us-currency-... http://elliswinningham.net/index.php/the-basics/us-currency-...

Also maybe:

http://bilbo.economicoutlook.net/blog/?p=9281

A rant on deficits:

http://elliswinningham.net/index.php/2017/01/10/deficits-mat...

For more, just look up modern monetary theory.