Why is it no matter how much storage I buy my iPhone still full smh

You are buying iCloud Storage space. iCloud does NOT expand the internal storage available for your device. It is meant for syncing and backing up of your device. Not to install additional apps or music to your device. iCloud CAN save you room on your device if you are using certain services correctly and have enough available storage on your phone for them to do their job correct. Case in point, since you have iCloud storage, do you have iCloud photos enabled? If so, use the optimize storage function. This stores a thumbnail of your pictures on your device instead of the full image, and if you select it, requires a little WIFI or cellular data to buffer the full image in for viewing, then its buffered back out, just like a web page. I have a 50gb+ photo library, and it only occupies 2gb on my phone as a result. But that requires the device to have room to think. If you are already nearing or at the limit of storage, remove a couple inconsequential apps that don't have any losable data, Facebook, messenger, Netflix, Hulu, and so on, let the device manage its storage and photos for 24-48 hours and then you should be fine.

You must delete some more. It's the only way to free up space.