well how did we work out water wheels? we see a flow, we stick something in the way so that the flow moves it.
A volcanic vent is a gushing source of fast moving gasses. stick a turbine on top.
Same technique as such archaic devices as windmills, water wheels, steam engines....just about every energy source we have used for mechanical purposes has used some form of turbine up to the latter half of the 20th century.
so how did we work it out?
Stone or wood does the job of early machines, metal is only needed in more advanced high-stress versions.
Many metals can be purified by reduction: you put them with a better reducing agent and out comes your metal. Alternatively, those volcanic vents are hot, providing all the energy you'd need for more conventional forms of extraction.
It's not like they'll have all this stuff available from the start, but remember that humans didn't have much more than sticks and stones to begin with.
And we didn't first extract metals in the way we do today, because we didn't have coal or oil back then: we didn't have access to it. Nor did we have blast furnaces.
The geothermal idea is actually more likely to work as certain metals are found in their pure form: precious metals like Gold, silver, platinum etc (all of which would work for making the geothermal plants)
Should the worst come to the worst, it's not to hard to evolve oxygen, even underwater. There's plenty in the water already, hence how fish are able to breathe through their gills!
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