What gives congress the money to spend on defense and public welfare?

Taxes layed and collected gives Congress the money to spend on defense and public welfare.

Explanation:

This is clearly explained through Article 1, Section 8 and Clause 1 of the US Constitution. The Clause states that Congress is allowed to lay, levy and collect taxes, but only for the purpose of benefit and prosperity of the United States and to provide for the overall debts of United States and the Defense and Welfare of the general public of the United States.

This included every form of taxes, duties, excises and imposts. Thus, it is clear from this clause that the Congress utilities its powers to access the taxes given by the public and utilities it for the defense and public welfare.  This clause is also sometimes called the Spending Clause.