What determines the pertinent regulations on the sale of products and services to passengers in international flight?

I am conducting research and have been looking at the Chicago and Tokyo conventions with respect to which convention governs the sale of goods and services to passengers when in international flight. Specifically I am interested in what determines which countries rules regulate the provision of alcohol or internet services inflight, which would determine the suitability of sale of the service / the permissible age of purchasers, and the ability for the airline to provide the service throughout the duration of an international flight as the plane moves through different national airspaces and therefore differing national rules for those services/products.

I am not sure which is the governing convention, is it the Tokyo Convention and as such the regulation of the country of plane registration (flag), or is it the Chicago convention and as such the country of overflight unless in international airspace when the Tokyo convention applies, or a completely different regulation? I have also seen reports that the governing rules could be those of the country of takeoff or landing. There seems to be confusion in the market on this, with airlines believing they are able to make their own rules, with most following their flag countries rules, but not all. In relation to alcohol and the age of consumption, the practice from airlines seems to be that the Tokyo Convention applies with the age of consumption reflecting the law in the country of plane registration.

I suspect that the Tokyo convention applies with respect to these services and products once the plane is in the air as otherwise, airlines would not be able to provide a consistent continual service, is that correct?