Fortresses are made up of multiple parts. This means that they should, like bastions, villages, and ocean ruins, have their own folder that contains the NBT data of how the structure is generated. For the other three structures, this can be found by going to .minecraft\versions\1.16-pre2.jar\data\minecraft\structures(structure name). Since fortresses have multiple substructures and various rooms, they should have a folder there, but they don't.