If instead of flying somewhere I traveled there by foot, I would surely use a lot of energy as well. Not just food and breathing, but also cooking, heating of the BnB I am staying in.

Surely you could say that this would be much less than a typing continental flight on a jet (you are probably right) and at that point the human CO2 can be considered a 0. But what exactly is that 0?

If you assume human produces exactly 0 CO2 all relative comparisons become basically infinities. It's infinitely better to walk than to cycle, it's infinitely better to walk than to fly, etc. But it's not really true. It may be several orders of magnitude better, even - but how many exactly?

I literally never saw this in the research in regards to CO2 produced.