From the Introduction: "The federal budget sustains government functions and plays an important role in shaping policy decisions. In practice, the process for developing and executing the federal budget is multifaceted. The Constitution vests Congress with the power of the purse, with provisions that refer to congressional authority to levy taxes, authorize the issuance of debt, and make appropriations to fund the federal government. The Constitution does not provide an explicit role for the President in the budget process. Rather, the executive budget process exists primarily due to statutes enacted by Congress, and it specifies roles for the President, the Office of Management and Budget (OMB), and executive agencies. The executive budget process is a complex set of activities that includes (1) development of the President's budget proposal, (2) submission and justification of the President's budget proposal, and (3) execution of enacted appropriations and other budgetary legislation. While some of the activities must be completed by specific dates, many follow a more flexible schedule established by formal and informal rules and procedures. After the submission of the President's budget request, each executive agency bears the responsibility for justifying its budget request to gain approval from Congress. [...] This report focuses on the justification of requests for spending provided through the annual appropriations process (i.e., discretionary spending)."
CRS Report for Congress, R47090
Congressional Research Service: https://crsreports.congress.gov/