I feel like I should share these bugs that devs have never felt the need to patch to help even the playing field for newer players. Hypocritical of me for even mentioning fairness while running something of a spamline, I know, but idc.
-First, close all flights to that airport. This means that superterminals are only possible at airports you are only starting operations at, unless you want to close all of the flights you have at an existing airport. The other way you could build a superterminal without closing all flights you already have is to start a second airline in the same country as your first airline, but that second airline needs to be able to afford huge terminals, so unless you're doing this at a tiny airport with super cheap terminal fees then you would need to grind both airlines to a point where 200 gates is no longer enough.
-Build a 200 gate terminal, then immediately sell it
-Repeat until you have the number of gates you want. I.e. if you want a 1000 gate terminal, you need to build and sell 5x 200 gate terminals. Do note that there is an upper limit on the number of gate slots that you could have at any airport, being somewhere around 32k. This means that the "ideal" size to build for is around 600 gates.
-Buy back all of those sold terminals, and now you have something of a superhub at your airport.
-Have fun (optional)