Same thing happened when we wore our iWatches.
We would be away from phones/computer....talk about a product or a place...and then *magically* receive pop ups or emails about said product/place.
We ditched the iWatches and haven’t had the issue, since.
Yikes......scary.
I’ve worn my first generation AppleWatch every day for five years and I’m going to call BS on your claims. First of all, AppleWatches up until the cellular enabled third generation AppleWatch, did not work independently of the iPhone and never worked dependent on a computer. Secondly, the AppleWatch never listens continually and only listens when the user lifts the watch and speaks the phrase "Hey Siri" to trigger its attention. Finally, Apple does not sell advertising to any third-parties as you describe and even using my iPhone or computer, unless GOOGLE was involved with a webpage or I was using a GOOGLE app or service, no Apple product ever provided a third-party advertisement based on a search or conversation or anything else, and most certainly none through my Use of my AppleWatch nor have any of the numerous members of my family and friends who own and use AppleWatches ever reported such an experience, and some of them own third and fourth generation cellular enabled AppleWatches.
Frankly, with your calling the AppleWatch an "iWatches," which is a denigrating term used only by anti-Apple critics, I doubt you ever owned one much less multiple AppleWatches, nor stopped using them for the reason you stated.