What you did was, you bought STORAGE, not usable phone memory. Storage Only. Here are your options.
You can move pictures, video and music to the SD card or your computer. Once you have the files to be confirmed moved or copied, you can then delete them from the phone.
If this were my phone, I'd create a folder on my computers desktop, and drag and drop the pictures, music and video files from the phone to the folder on my computer.
Then before I took any more pictures/videos, I would set up Google Drive so that it backs up all media files on my phone when connected to WiFi.
Then I would setup the Samsung backup on my computer, to once a week/month, backup EVERYTHING on my phone (just like you should be doing now). This should keep most things backed up, so that you lose very little should your phone suddenly get lost, die or become destroyed