If you talk with a friend about getting a pet, your phone and by extension the ad company that receives your location data, knows you 2 saw each other.

If that other person searched for pet food, then you will also receive ads for pet food even though you "only talked about it".

Other example, you post you have a new job on Facebook (which a lot of people do). Facebook knows where you live because of location data, and they know where the company is located since it is public data. So they know how close those 2 are. If Facebook then looks through your photos and notices none of them have a picture of a bike in the last 5 years. Then you are likely to buy a bike in order to go to work. Thus you get ads for bikes.