

To save you some time: Google and Meta are monitoring the conversations using the device’s microphone
This is not happening. It is not something that would be possible to hide, there are so many different places this could (and would) be detected by someone with the right know-how (data/battery consumption in particular).
Try and pay attention to how many ads you get that are completely irrelevant to you. It will happen way more often than ones that match up with your conversations.
The process you’re thinking of is oxygenation, not oxidation. Oxygenation is the binding of oxygen to other molecules, oxidation is the loss of electrons. When the iron in hemoglobin oxidizes (from Fe2+ to Fe3+) it stops binding with oxygen, and if it oxidizes further (to Fe4+) it can start oxidizing other molecules in your body. Your body has enzymes to reduce the iron back to a reactive state, but antioxidants also play a role in reducing oxidized molecules.