Understanding why a US President (including Donald Trump) does not have sole authority to dictate the annual DoD budget is just as important as understanding that our President and Vice President are elected by the electoral college system and not by national popular vote results.
The US federal budget providing funding for various agencies each fiscal year is determined by legislation originating in the US House of Representatives, which then is passed to the US Senate, and then is submitted to the President who can either veto it or sign it into law.