Tax payers. In Germany, airplane fuel is not taxed.

See this Greenpeace study (in German) about how much cheaper flying is than train rides and why:

https://www.tagesschau.de/inland/regional/nordrheinwestfalen....

Not taxed does not mean it is subsidized. Subsidy is when money is paid by the government to the airlines.

Tax on fuel reflects the externalities to the whole of society, and arguably it is not high enough.

So exempting tax (equivalently not charging for externalities) is indeed a subsidy, a transfer of wealth from government (and so society) to the airline operators.

Regional airports are usually subsidized by local governments to put them on the map. Also jet fuel could be taxes at à similar rate as gas/diesel