A feature in Android version Marshmallow (6.0) and later that allows you to do this. However, not many phone models actually support it, so look up your model before going any further. There are also good reasons to avoid using an SD card as internal memory. If you do, you cannot ever remove the card, or your phone will start getting a lot of error messages and prompt you to reinsert it. Also, using an SD card for internal memory could cause the card to wear out faster and cause errors or slow down your phone.