Daft question about fermentation

[Keruso]

I use plastic fermenters with an airlock. When the wort is transferred and the lid goes on, for the air that is trapped in the headspace - does it get pushed out of the airlock by the CO2 the yeast creates, or does it get absorbed into the wort and get used by the yeast, or maybe a bit of both?

 

[Hazelwood Brewery]

Probably a bit of both. We know that yeast uses oxygen in the first phase of fermentation in order to multiply. I don‘t know how much is absorbed into the wort and used by the yeast but I assume (from years of fermenting brews) that whatever is absorbed is used by the yeast. Anything left in the headspace is probably pushed out with the CO2.