There isn't a single answer to this question. However as a general rule a baby born during flight is regarded as having the citizenship of the country in which the aircraft is registered. (It is worth remebering that the country in which a particular aircraft is registered might not actullay be where the airline is registered). By contrast the place of birth is conventionally regarded, that is information as included in a birth certificate, as the point of 'disembarkation' of the new-born !
The problem is that the place of birth might not have any bearing on (a) nationality or (b) even on citizenship. These two matters depend on the national laws of each country.
These conventions, now applied loosly to in-flight births, deive from the conventions on births at sea. Essentially the convention was that nationality corresponded with flag registration of the ship. It is however important not to confuse nationality with citizenship.