There needs to be a budget passed by Congress to allocate money to be spent. It hasn't been done in some time as Congress can only agree on continuous resolutions. The majority of the budget can't be changed, things like interest on debt, mandatory programs, etc. That's why Musk's BS is simply that.
Next is allocation, or spending. Congress has a pool of money for a division of the government through the budget and now allocates for a particular program or need. It then allocated this money and Treasury is given the responsibility to pay for it. When revenues don't match expenditure we borrow money from ourselves to take care of the shortfall.





Again, not Biden. Congress