Where does this liquid helium come from, and more importantly where does it go after it's been heated? Im guessing they release it into atmosphere, so is there enough helium to mine for this plane to go mainstream?
The helium is cooled by the hydrogen going to the engine.
You can't run the pre-cooler on the hydrogen directly, because the hydrogen infiltrates the welds (and everything else) causing hydrogen embrittlement, which will quickly tear the heat exchanger apart.