https://fiscaldata.treasury.gov/americas-finance-guide/federal-spending/

This site from the Treasury has great info, but some highlights:

22%: Social Security
14%: Medicaid and other Department of Health and Human Services spending (health research, the FDA, FEMA, the CDC, temporary welfare for needy families, foster care, other stuff
14%: Medicare

At this point, we've already hit half of the federal just on social security and health spending.

13%: National defense
13%: General welfare. This is things like SNAP, WIC, SSI, the earned income tax credit, some housing assistance programs, and other welfare programs.
11%: Interest payments on debt.
5%: Veterans Benefits

The remainder is divided a million different ways, but those are the largest categories. Social security and healthcare are by far the biggest expenses, and those costs are also expected to grow faster than the revenue that funds them, which is eventually going to cause a big problem that neither party has any interest in addressing. It should absolutely be noted that that healthcare spending is disgustingly inefficient due to the disaster that is our healthcare system, and it could be much much more efficient. But basically, a huge chunk of the money goes towards retirees. It's probably a total coincidence that those are the ones that vote the most too.

I've been slightly loose with the categorizations there in order to not get bogged down in some irrelevant classification details between spending functions and government departments, but the general picture is accurate.