The federal budget is legally originated in the house of representative. The details of this budget can be literally anything. They can say that only agency's that start with the letter A are funded, etc, it doesn't matter. They could zero out the military budget if they wanted. They could pass universal healthcare if they want. It's an appropriations bill of a sovereign nation, there aren't any rules. This bill in whatever form, then goes to the Senate where again, it can be passed with a simple majority.

Yes the rules of the senate allow a filibuster to exist, but any bill can be filibuster proof with a simple majority of the senate. There is no constitution guarantee of the filibuster. The only reason the filibuster exists is because the rules of the senate (which only require a simple majority to change) allow it to exist.

Any funding bill can say, fiscal government spending will stay constant year over year, until a future funding bill changes it q.e.d.