There's a creepy video gaining a lot of views, it's a live test that "proves" that Google is always listening to your mic
Maybe Google is, in fact, hearing all conversations near our phones and computers, but that is unlikely (but totally possible) because it would be like streaming a podcast all day, therefore a lot of bandwidth and battery would be lost. Another possibility could be that audio is processed offline and then just the text is sent to Google but (most of) current mobile phones are not capable of that just yet.
Here is another experiment with results that contradict the other one
So maybe there is a simpler answer
The Baader-Meinhof Phenomenon
Is a psychological phenomenon describe as
Baader-Meinhof is the phenomenon where one stumbles upon some obscure piece of information—often an unfamiliar word or name—and soon afterward encounters the same subject again, often repeatedly.
Source and see more
Google ads showing things that we recently talk about might be just normal and because our brain is now spending more attention on that subject.
In my own experience, I was talking about a brand that is not really known in the subway and then I saw an advertising poster about it. Clearly, no one was manipulating those advertisements because it's absurd to change a poster to match the interest of one person on the subway.
I am not sure if Google or Facebook or another corporation is listening to our conversations but I believe that it's unlikely and it's rather a psychological effect.