I am curious what kind of plasma is used, as there are many ways to create plasma under various conditions, with very different efficiencies, selectivity, yield, etc.

Among the worst is the Birkeland-Eyde process, hot plasma by electric arcing, like it happens naturally during a thunderstorm. Even microwaving the air is more energy efficient. And amongst the most promising are transient non-equilibirum / cold plasma approaches using ns pulse generators.

Edit: So according to this video "Green Lightning" is indeed using the Birkeland-Eyde process. In other words, there is no way they even come close to their energy efficiency and yield claims. https://youtu.be/8TOnyAwJyvg?si=D9hat_XAfvuAAmpB&t=165