I've noticed that many people confuse discretionary and mandatory federal spending. An easy way to remember the difference is that discretionary spending is determined by current legislators via the appropriations process and mandatory spending is required by exisiting laws. For example, the military's budget is revised each year based on our perceived defense needs, whereas payments for social security are automatically calculated based on a formula that is specified in the laws that legislators passed dealing with the provison of social security benefits.
Here is a breakdown from OMB, which includes both discretionary and mandatory spending. https://www.nationalprior...
You can also explore where the money the federal government is spending comes from here. https://www.nationalprior...
The National Priorities Project writes about the federal budget in good detail here. https://www.nationalprior...
