Well that first number, the number it advertises as having for storage, is not what you have access to. It never is for any device.
You have to remember that that is the full storage number. How much it can hold, total. Not, however, how much you would have access to. This is because the OS (Android in this case), any launcher that came with it and any preinstalled apps all use up part of that space. So the only space you have access to is what is left over.
You can try uninstalling any apps you don't need/use, if they can be uninstalled. Some preinstalled ones cannot be and can only be disabled. Otherwise, other than adding a SD card, which will give you "External" storage. Or if the device will allow converting a SD card to "Internal" storage, not all devices allow for this, then there is no way to add "Internal" storage.
The following is how you turn a SD card from "External" to "Internal" storage...
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).
Go to device “Settings”, then select “Storage”.
Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
Now select “Format as internal”, and then “Erase & Format”.
Your SD Card will now be formatted as internal storage.
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.
Go to "Settings", and then select "Storage & USB".
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.
Once this is done, reboot the device and you can start running things from the card.
IMPORTANT NOTE: If you don't reboot the phone, some things may not work correctly, so make sure you do the reboot.
An Additional Note: Some apps (like system ones) are not movable. So while you can move downloaded ones, and some of the system ones, you will not be able to move many preinstalled apps.
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.