First, you need to be using a phone that is running at least Android "Marshmallow" (6.0) 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. If that is the case, you will not be able to use this information.
How to turn the "External" SD card into "Internal" storage - Option A
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 - Option B
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.
NOTE: If you don't reboot the phone, some things may not work correctly, so make sure you do the reboot.
A Final Note: If you set up the card as "Internal" storage, then the data on it will not be readable by any other device. To access the data you would have to connect he phone to say a computer and copy it over to the computer from the phone/card. Otherwise only phone can read the data. Also, be sure you do not reset (hard or factory) the phone. If you do, then all the data on the card will no longer be readable, even by the phone itself.
The only reason for converting the SD card to Internal in Marshmallow was because that OS did NOT recognize SD cards as Portable Storage---or at all. Moving stored app data was simply not an option. Nougat gives back the option whether to convert it or not, and Oreo has SD card storage as portable-only---for good reason. I see a lot of posts with users who cannot access their internally formatted SD cards' data in their new devices. This can be a big problem if your phone or tablet was the only device you had for your valuable data, with no access to a computer or the cloud. You see, when you reformat to "internal," you not only lose what was on the card, but you lose that card's new data permanently should you upgrade. This happens because your original device encodes and locks the card to be read by only that one. No more family photos, important documents you might need in the future (like income for taxes or benefits) or favorite tunes. Gone.
Back to your device, some apps may permit moving to SD, while others do not. Apps designed as utilities or widgets are designed to add functionality and generally need to be run from internal storage becuse they need immediate access to what's there.. Photo galleries, office and music apps generally allow moving downloaded or created content to the SD card, allowing access when needed. The functional portion of the app will remain internal. It's just the way Google set up the way your phone handles what it has. The way to find out movability is to open "settings," opt for "general" and open the setting for "apps." Select and open the one you might want to move. It will show the amount of storage and data used by the app. Select and open that subfile. If the app is movable, there will be an option marked "Change." If no option, you won't be able to move it.
Actually, in all versions of Android since "Marshmallow" came out, the option to make the SD card has been part of the original OS. However, many manufacturers have decided to remove this option. Why, can't say, but they do actually do it.