Stan Meyer was using hydrogen generated on board the vehicle in the vehicle's internal combustion engine. There is more to exactly what he was doing that involved resonance but I don't know very much about it.

Most of today's hydrogen tech uses hydrogen in fuel cells, which have no moving parts (except fans to blow air and H2 through the stack). No combustion with fuel cells, just a chemical reaction.

Greek physicist Petros Zografos is doing something like Stan Meyers did, PBS broadcast a segment on him and then pulled it, saying they should have been more "skeptical".
https://www.youtube.com/watch?v=aXHWentESoA

More about the pulled PBS segment: https://www.scivillage.com/thread-3178-post-9252.html

These guys are (or were, in Stan's case) on to something, I'm convinced. The Wiki page on Stan's death is interesting. This tech will disrupt a huge chunk of the status quo so it isn't surprising that there are shenanigans going on.