It is... very complicated. Strictly speaking, if congress doesn't pass a budget the previous year's budget is in effect for some things, and others can be hand

It is... very complicated. Strictly speaking, if congress doesn't pass a budget the previous year's budget is in effect for some things, and others can be handled at the level of a 'continuing resolution' that doesn't require the president's signature. Realistically all parties (house/senate/exec) generally come up with a deal, which is sometimes a proper set of spending bills, and sometimes a hack-up of short term fixes.