[wahahah]

How do they get the CO2 out of the sub?

[mdekkers]

CO2 scrubbers

https://en.m.wikipedia.org/wiki/Carbon_dioxide_scrubber

[sbierwagen]

(Which dumps the CO2 overboard, which is why the sub needs to constantly replenish the amount of oxygen in the air.)

[CobaltFire]

Specifically (at least in the US) amine treatment using Monoethanolamine:

https://en.m.wikipedia.org/wiki/Amine_gas_treating