15 GB Google warning refers to capacity of your CLOUD storage (Google Drive) for backing up your phone provided for free by Google. It is up to you what you want to back up there. If you don’t need backup of your photos, In Google photo app go to your profile (circle in top right corner), click on backup option, turn it of. This will free up your space storage in Google drive cloud. There will be warning if I am right that photos from Google drive will be deleted, but this doesnt affect photos stored in your physical phone.
Btw to provide only basic (smaller) storage of cloud for backing up settings, contacts, emails etc, it is standard model for all - MS (OneDrive), Apple (iCloud), Google (GDrive), Adobe CC… And if you need more online/cloud storage, you have to pay, in case of GDrive 2€ for 100gb monthly…
Another way to back up your photos, emails etc, and I would say more safe in terms of personal info, is to your computer via usb or buy otg capable usb memory stick…
