On my new awesome s9 phone, and also being new to android, I was recently at a concert and I filmed the singer for about three minutes and took maybe 5 photos. We went back to the hotel after, and when I woke up the next morning, I had notifications from Telus saying I used up all my 1 GB of data. After some playing around through the settings, I realized that my google photos was set to upload over data, as I didn't have it set to only wifi. I still can't believe that just a small amount of video would consume 1 GB, but it did so I had to purchase another GB for the rest of my month.
Yeah it's a good idea to check that none of the apps installed have permission to upload to the cloud - Including Samsung cloud which is set to backup by Default - which I think is wrong - No apps should be setup to backup to any cloud storage by Default! 🐧