I see these all the time too.

1. I bought one of those spinning face brushes for my girlfriend. A few minutes after purchasing online, I started seeing advertisements from that same store, for the same exact brush. Literally 90% of the page views were showing that advertisement.

2. I was browsing Airbnb accommodation for a trip to let's say Mexico. I was checking apartments on and off for weeks. I didn't see any Airbnb advertisements during that time. The minute I book that accommodation, Airbnb starts showing advertisements for rooms in Mexico.

3. I'm browsing barbers in a new city. No advertisements until I book an appointment with one online. Then, I start seeing advertisements for the same barbershop. Now, that has potential, but the advertisements stopped after a week. Why do I need to book a second hair appointment in the same week? Why not recognize I booked a men's haircut, and start showing me ads in a few weeks?

For all the tracking, privacy invasion, and fancy "machine learning", advertising sure is dumb.

A little over two years ago, I Googled the name of a local Dodge/RAM dealership (on my iPad, thus no ad blocker) so I could browse their selection online before I ventured over there. That afternoon, I bought a new truck (from that dealership).

For almost A YEAR afterwards, I would constantly see ads about RAM trucks and various dealerships in my area (including the one I Googled and bought the truck from) when browsing the web on my iPad. Not every ad, of course, but certainly often enough that I would notice it a couple times a week and just laugh and shake my head at the dealerships who were just throwing their ad dollars away.

My guess, some people buy a mattress and end up unhappy with it, return it, and buy something else. This is enough people that it's worth showing mattress ads to all recent mattress buyers.