How to make ethanol:
1: Use diesel and gasoline powered farm equipment to grow and harvest a sugar-rich crop.
2: Use fossil fuel supplied heat and electricity to operate a cooker, which turns the plants you grew into cattle feed and ethanol.
How to make hydrogen:
1: Obtain methane from fracking or some other fossil source.
2: Process the methane to produce hydrogen.
Both processes use a large amount of fossil fuel, thus releasing a large amount of CO2 into the environment, to make a high density but inherently inefficient, potentially very useful energy source.
How to make green ethanol:
1: Use only electricity to run the farm equipment and ethanol cookers.
2: Make all the electricity using wind and/or solar.
(3 optional: You can use some of the green ethanol to run some of the above two processes.)
How to make green hydrogen:
1: Use electricity to power electrolysis to separate water into hydrogen and oxygen.
2: Make all the electricity using wind and/or solar.
How much of the ethanol we use today is green, compared to the total amount of ethanol? Zero%
How much of the hydrogen we use today is green hydrogen compared to the total amount of hydrogen: 0.04%
Right now essentially zero of the hydrogen used to send rockets into space is green. There is a move to make some of that as green hydrogen. Given the example of ethanol, there is no reason to believe that green hydrogen will ever be the main fuel supply for any ordinary earthling transportation system. However, I’m willing to give it a chance. Let’s first fuel our entire global rocket industry with green hydrogen (wherever hydrogen is used). Then, also, let’s create all the other hydrogen used in industry for things other than transportation (there is quite a bit of that) using green technology as well. Once we are making green hydrogen for all the hydrogen uses, then we can talk about hydrogen being used to fuel some percentage of our transportation industry.
Having said that, hydrogen is of limited use in this area, since fuel cells are inherently inefficient and for other reasons. There is no reason to totally write off hydrogen, but we have every reason in the world to totally write off non-green hydrogen.
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Fuel cells are at least as efficient as large, high weight-to-power-ratio (ship)diesel engines.
And brazilian not-so-green sugar cane ethanol is still somewhat better than just running trucks with the diesel fuel used by the agrarian equipment to produce said ethanol.
They are close to the efficiency of combustion engines, nothing close to electric motors.
Another use for hydrogen is to replace coke in steelmaking. A Swedish company has a small-scale prototype working. Commercial use will of course require large quantities of hydrogen. Replacing coke with hydrogen will have little benefit if the hydrogen is not green.
Dr. Sabine Hossenfelder has an excellent overview of hydrogen on her YouTube channel.
https://www.youtube.com/watch?v=Zklo4Z1SqkE
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You cannot ‘replace’ coke in steel production by hydrogen, because the carbon from coke is what makes steel from iron.
Of course one possibly can reduce the amount of coke needed.