"they" may have claimed to make a car run on just pure water but...

"they" may have claimed to make a car run on just pure water but its only click bait, I recall one in the 70s backed by Joh Bjelke-Petersen before click bait was even invented. Elon certainly has not even claimed to have done it, in fact he has publicly said that Hydrogen is a waste of time and more importantly, energy.

You have obviously had nil science or physics in your education and critical thinking seems to be severely lacking as well. The only way a car can run on just water with currently available technology is if you put energy into it first which effectively means that it also has to carry that source of energy.
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For example heat it up to drive a steam engine, the Stanley Steamer was probably the most famous example but trains used the idea very successfully for over half a century. Or feed electricity in to split it up into Hydrogen and Oxygen which can both be useful components for mobility in a number of ways. There maybe a way to utilise water in a fusion reactor but humans have a way to go before we can contain and control energy from fusion so putting one in a car is a few years away yet.