The terms tell the difference between them themselves actually.

National means domestic, local, internal or some ways, federal. It means it covers a nation/country/region’s domestic/internal/local affairs and happenings.

When you go global, on the other hand, you are talking about collective worldwide economic ongoings eg an open international comprehensive market every nation participates in or its’ interest and concern are factors. In short, global economy covers the status or growth or welfare or wellbeing or activities, as all nations have part in it or interest/concern or impact in and from. And a national economy of a nation/country in many ways, is, to some degree, subject to ongoings in global picture. Prices, money, output and input, resources, etc in the former,in many circumstances and scenarios, can affect, or be affected by what is going on in the latter.