Yeast and open fermentation

I've been reading a bit about yeast biology/chemistry and it seems like in the presence of oxygen (aerobic respiration) the yeast does not produce ethanol as opposed to the closed anaerobic fermentation in which it does produce alcohol. Given that, why with wine making do you generally start with an open fermentation and then switch over to closed when SG is around 1.020? Wouldn't the alcohol content be much less than the alcohol calculated from the OG ~1.090 since much of the fermentation was aerobic?

There are two things I've read that may allow the yeast to operate anaerobically. The first is that the yeast can operate in it's own micro-anaerobic environment even in the presence of oxygen. The other is that I know a CO2 blanket is formed over the must in the open fermentation container, does this possibly prevent as much oxygen interacting with the yeast thus making it operate anaerobically?

the second is exactly right, the co2 drives of just about every bit of oxygen during active fermentation. The reason you would rack the wine to a closed container at around 1.020 is that the fermentation is slowing and air movements in the winery might be enough to wash away the CO2 blanket faster than it is produced.

Yeast consume most of the dissolved oxygen during the very early stages of their cycle, when they are prepping their cell wall and getting their metabolic house in order which leads to the exponential growth phase and ethanol production.

Once ethanol production starts, massive amounts of CO2 are produced and even in an open fermentation there is virtually no dissolved oxygen remaining, therefore the yeast are operating anerobically.