Er...well no, they most certainly aren't magic. They're fictional super powers yes, but the point of science hasn't just disappeared from the verse entirely. A lot of quirks (Not the one example you can provide since they can still take liberties as its fiction) do in fact rely on a lot of scientific bases. By you acting as if science isn't at all a factor (Energy transfers like Shoulder jets quirk is based on jets, and is a rather simple topic since its just fuel provides energy), then whats the point of making calcs and basing them off real life laws of physics? That wasn't too long, dont worry. I mean, just look at Momo. Her quirk literally relies on the knowledge of science. Its a concept in MHA, just able to be loose because of fiction, but nothing goes against the classic 'every organism needs an energy source'.
If there was some abnormality to do with High-Ends quirk that needed to be known like you suggest, it would be stated. Otherwise, lets just assume High-End can't breathe because its never been confirmed.
The fuel is probably whatever High-End uses for his supply of energy. I'm guessing food and drink since thats the default, he still has a mouth, so yes, orange juice would count i guess.
Because that would be stated if High-End got his energy through means like this? Much like the many emotion based quirks like Love and Stress. If not, we assume like we always do that unless they're unlimited sustenance, they use consumables. Iida also requires food to use as fuel for his engines, so i have no idea why this wouldn't be similar, this quirk was originally from a human after all.
It not fire based, its just combustion. Thermal energy is what is used within jet engines in order for the fuel to convert into whatever energy source needed, usually kinetic. This is why Jets get hot. They aren't water jets quite obviously, nor do air jets produce a short burst flash of light like the panels suggest. Do you know what does though?
