‘How are we going to pay for that?’

The same way that Congress pays for anything it has the votes/political will for. The house and Senate agree to spend money on a bill/project and the President signs the bill. New money is created and dispersed in the private sector (no, taxes don't pay for anything) to deploy the resources necessary. Numbers are changed by keyboard entry and accounts go up or down at the Fed.

There is no need to "find" the money because the US uses a sovereign fiat currency that is created as needed. Money is the easy part, as it is available in infinite supply, regardless of present or past revenue (an oxymoron for the currency issuer). There is no "infinite+1" in math that would better enable spending if only more taxes could be collected first.

The real challenge of budgeting spending is assuring that the real resources needed, that the money is intended to deploy, are available, not "finding" money that Congress creates on demand. Even Allen Greenspan admitted to this under oath when pressed about the future of Social Security. Numerous others, including Bernanke, Cheney, Bush, and even Trump have publically admitted that "We make the money"; or "Deficits don't matter".

Congress can "afford" anything that is available and priced in US dollars. We could, at any time, enjoy the nice things that most other countries take for granted as a right of citizenship. All we are ever lacking is the political will in Congress to go against the best interest of the donor class that presents a standing offer for any representative to become relatively wealthy by simply playing the game.

Articles like this that make either/or comparisons between what we spend on and what we "could have" instead are playing into the limited money propaganda. We can have butter and bombs if we want both, although I believe most would reject unlimited spending on bombs for its own sake if presented with a real honest choice. The war machine simply has a better PR department than does the dairy industry.