

They explain on their site here.
There is membership/assessed, which goes by each member country’s wealth. There are also voluntary contributions, which comes in a few forms - core, which have no specific requirement on use, strategic donations which have general goals and can be applied to anything in that field/theme, and specified, which go to specific projects or efforts.
There is also a budget portal which details how money is spent.
They sure are trying to.
https://www.flhouse.gov/Sections/Bills/billsdetail.aspx?BillId=81923
Some highlights: