I usually wear the tin foil hat in these debates, but I must concede in this case: the eavesdropping phone theory in particular is difficult to substantiate, from a technical standpoint.
For one, a user can check this themselves today with basic local network traffic monitors or packet sniffing tools. Even heavily compressed audio data will stand out in the log, no matter how it’s encrypted, streamed, batched or what have you.
To get a sense of what I mean, run wireshark and give a wake phrase command to see what that looks like. Now imagine trying to obfuscate that type of transmission for audio longer than 2 seconds, and repeatedly throughout a day.
Even assuming local audio inference and processing on a completely compromised device (rooted/jailbroken, disabled sandboxing/SIP, unrestricted platform access, the works) most phones will just struggle to do that recording and processing indeterminately without a noticeable impact on energy and data use.
I’m sure advertising companies would love to collect that much raw candid data. It would seem quite a challenge to do so quietly, however, and given the apparent lack of evidence, is thus unlikely to have been implemented at any kind of scale.