We are going to need low cost green hydrogen. Existing electrolyser technology uses precious metals and so they are expensive/current limiting factor. However there are lots of startups working on bringing to market electrolysers that use non precious metals.
In terms of chemical processes, fermentation is quite simple. In addition to the bioreactor you need a device to control the gas inputs and you need some downstream processing (product extraction). For ethanol distillation is most commonly used but there are newer lower energetically demanding techniques that can be used for extraction. Not all products can be distilled out though, so extraction is somewhat product dependent.
edit: some interesting hydrogen companies
https://www.sunhydrogen.com/technology (like a solar panel but for hydrogen, uses light energy to split water into H2 + O2)
https://www.alchemr.com/technology/ (electrolysers that use non precious metals)
https://www.h2pro.co/technology (membrane free electrolysers)
There are also several companies developing 'turquoise' hydrogen, which is a plasma based technology. I have no connection to this website, but the first few paragraphs it lists a few companies in this space: https://www.h2-view.com/story/four-more-technologies-for-tur...