They are submitted to Congress for approval by the administration. When the President and Congress disagree, some of the bills are not approved and certain government operations are shut down. This happened in the 1996 fiscal year. In fact, the budget for fiscal year 1996 has been approved only after the 1997 budget was. In the case of such a deadlock, stop gap budgets are passed by Congress to allow the government to continue to function until a final budget is positively voted on.
Budget are acts of humans.
They represent hard data implausibly coupled with aspirations, projections, goals and hopes. They are prone to mistakes, greed, cronyism, ulterior motives. The existence of a mechanism to amend budgets is, therefore, of the essence and to be greeted. A budget amendment is often ceased upon by the opposition as proof of the government's fallibility and failure. But in a changing world - they who do not adapt through change are doomed.