Since January 2019 the presidency of Venezuela is disputed with 57 countries recognizing Guaidó as president and the rest of the countries either supporting Maduro, the national assembly or being neutral. In this context, who decides in Venezuela the annual budget, import taxes, and the usual tasks of the executive power? Or are even these tasks splitted somehow in the country now between the 2 factions?
International recognition is of secondary importance here. What really matters in a power struggle like this is the military. While there are some in the military that would support Guaidó, the main part remain loyal to Maduro.
This allows Maduro to be the de facto president. He has been able to introduce price and currency controls, for example.
Normally it would be the assembly to propose a budget, but much of the basic function of government is not operating at the moment. It seems that Maduro's government is able to raise money (for example by selling oil) and isn't depending on the national assembly pass a budget, since the national assembly has been ruled unconstitutional. This is what I mean by a non-functioning government.
There is not much point looking at the written constitution or the opinions of world leaders. This is now a government that exists as a result of support from the army, not as a result of a mandate from the masses.