The place I see the most unsettling ads (that seem to be driven by overheard conversation)
There’s a simpler explanation – you’re in the same geospatial region or you’re connected to the same networks as the people you’re having conversations with, and those people also looked up the things they have conversations about.
If you have GPS, Wi-Fi, or (possibly) Bluetooth, then that’s how they can pretty easily associate you to those people.