The previous administration, or President, a budget. The previous Congress that budget. And what Congress approves can be radically different from what the President proposes. What people forget (or never knew) is that the first year of every presidential term starts with a budget approved by the previous administration and Congress.
Note as well that President Reagan didn’t just go along with the wild spending binge of the previous Democratic Congress for fiscal year 1981 when he came into office on January 20 of that year. Almost no one remembers now the much vilified at the time 1981 Reagan budget cuts, his first major legislative initiative.