iPhone 6Plus and iOS 15.7.9 Storage issues

I had a iPhone 6s for years and just now recently bought a new used iPhone 6plus and both had the same amount of storage so I did the quick start to transfer everything off my old iPhone to my new iPhone and now my new iPhone 6plus is saying that my storage is full and I already offloaded most of my apps and it’s still saying my storage is full even whenever I updated my apps it’s saying that I don’t have enough storage to download it but I already have it downloaded I’m just updating it and a customer service person of apple support said if I backup my iPhone and write down my password and apps that I can reset my iPhone 6 Plus and won’t have anymore issues with my iPhone is that ture and I would have to turn my iCloud Drive off or it will be deleted also I haven’t done it yet but I’m going to because something keeps popping up my storage is full and it’s makeing me really upset