TL;DR Some cellular cetwork providers triangulate your location and sell it to god knows who. And since real anonymous data is a lie, those who buy that data probably can tell who and where you are. Check if you can opt-out.
A while ago I went on a trip to Hamburg for a few days and I kept wondering how these bits of advertisement that keep comming though, even though I block pretty much everything, knew my location and just started recommending stuff in Hamburg. I did not share my location and only rarely even used any kind of tech those days.
So, I did a bit of research and stumbled upon an article (german, sorry) which basically said that the provider Telefonicá sells 'anonymous' location data of their clients.
Telefonicá not only has their own service, but also offers their service to Vodafone and PremiumSim for example. And no, it's not the data you give out by activating GPS tracking. They locate you by cellular triangulation. They of course do say, that the data is anonymous, but companies buying this kind of information are likely to already have a lot of other information and are able to conclude who this location data belongs to. Additionally I did not check if it is really location data only, but it wouldn't be hard to package that data with for example data about your phone: https://android.stackexchange.com/questions/133427/can-cellular-network-providers-detect-the-phone-model-make-when-their-sim-is-act#:~:text=Network%20provider%20also%20accesses%20your,handset%20is%20valid%20to%20use.
Basically by simply having a contract with them or their partners, I accepted this. You can opt out. But it's not like anyone tells you about this.
For the folks in Germany - just send 'Abmelden' via SMS to 66866. (On this note, don't forget to trust no one on the internet and always double check these kinds of information. =)
For other folks - Sorry, didn't research much further.