You have raised an issue that is being discussed but unclear.
Are the uncontributed funds in question direct contributions to NATO or individual nations defense budgets calculated as a percent of the specific nations GDP.
I have come to believe that it is the latter. That leaves open the question about sources and amounts for the administrative NATO budget.
The administration of NATO is billed to all members of NATO. Thats around 350 million dollars per year. Other than that no money changes hands. Each country funds it’s own military the way they choose to.