Just because most adtech is the equivalent of Internet billboards on the side of the highway doesn't mean these systems aren't in place. You don't even need a very complex system when the entire device platform is designed to spy on you.

Take a look at this for example: https://www.theverge.com/23293687/amazon-irobot-acquisition-...

Even simple ad systems are a huge privacy threat. Add to that audio, location, and even AI screen reading surveillance, and you're screwed.