If you are just wanting to move files (not apps, as that is a whole other story) then there are a couple of ways to do it. One, you can locate the file manager on the device (or download a file management app) and move the files from the "Internal" Storage to the SD card using that. Then, to make sure any of the photos you take are saved to the card you will need to go into the "Settings" in the camera app (or whatever app you are taking pictures through) and set it to save to the card. The same goes for any app you are downloading music, photos, etc. (again not apps themselves) through. You would need to change the "Settings" in any/all apps downloading through to save to the card. If a particular app won't allow for saving to the SD card, then you would have to look into another app that does give you that option.
Now if you are talking videos from say Netflix, those won't save anywhere but the "Internal" Storage. They set it up that way. And if you are talking saving apps to the card and running them from there, then you would have to see if the following would work on your device. NOTE: If you choose to do as listed below, make sure you do it before moving any other items to the SD card.
First, you need to be using a phone that is running at least Android "Marshmallow" (or newer) or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your phone, or they don't work for you, then you have a phone that either has an older Android version or has been restricted by the manufacturer.
How to turn the "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.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the phone, some things may not work correctly, so make sure you do the reboot.