Your phone isn't secretly listening to you, but the truth is more disturbing

I get all the proximity-based aggregation, and creating graphs of relationships to leak content between personal "algorithms" (dislike that wording but that's the colloquial usage), and tracking between sites + social networks, and all the basic stuff ... but can somebody explain how I immediately get served ads relevant to text typed into (presumably-encrypted) iMessage conversations?

I also have a couple distinct memories of getting served ads for products I've never searched for or never bought before, after I either bought it in a store or, even weirder, literally just picked it up, looked at it, and put it back on the shelf in a store?

I can craft some kind of super-surveillance-state theory as to how you could achieve that, but it feels very unlikely to be deployed at a small CVS lol

Anyways, these might just be coincidences but still perplexing to understand how it's done.