There is growing evidence that long covid is actually CIRS (toxic mold usually from living in water damaged buildings) triggered by covid. Only 25% of people have the genes for CIRS, but then it takes exposure and a immune shock to get your systems out of whack (lyme, covid, etc.) The vast array of symptoms come from the mycotoxins becoming present in every part of your body and affecting all your systems, especially your gut biome.
Source: I have it and am on the long road to recovery. Before I moved out of my house this spring, I could have days with several migraine level headaches and where my vision was so messed up I could barely see 10 feet. I don't have bad days like that anymore since I moved to my sisters. (I have lots of other symptoms too, this is just a sample.)