Well, most things are set to save to "Internal" storage and won't save to a SD card unless the specific app is set up to save things to the card (like say photos set up in an app to save to card rather than device). However, apps themselves are a very different story. There are two things necessary to make that work, save apps to a SD card and run them from it. 1. Your phone has to be running at minimum Android "Marshmallow" and 2. The phones manufacturer has to have not restricted this option on that device. Sadly, they do sometimes.
If the phone is running "Marshmallow" then you can try the following and see if it will allow you to make the change. If not, then your best bet is to save Photos, Videos, Music, and similar files to the SD card rather than the phone to save on space.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.