When a person is getting unwanted ad's I suggest to put the phone in Safe Mode which disables all the downloaded apps and leaves the stock ones working which can help determine if this was an app.
If it stops in Safe Mode then it's a case of uninstalling the apps one at a time.
The stock apps do not typically have ad's written into them.
A person does not have to visit adult / streaming sites etc to be hit by adverts.
It's always beneficial to be wary if sideloading via an Apk file which is why the phone blocks these out of the box and has to be circumvented by the owner of the phone to allow them.
Yes the iPhone you mention is an impressive device which just does things simply however for me iPhones and iOS is way too sandboxed for my liking and I'd miss the freedom to have my phone the way I want it and not the way Apple wants me to have it set up etc.
Each to their own.
What you experienced is something I have never seen myself, its a result of installing some "bad" apps and the only way around it is uninstalling the app that did it. Worst case scenario it can be solved by a factory reset. Its indeed a downside that comes with having freedom over your phone (as opposite to an iPhone which is a closed platform).
Hopefully our advice will help with allowing you to enjoy your phone again. The S9 is a marvelous device and its a shame to be thrown away due to some bad apps.
