How to Set an SD Card as the Default Storage Device on Android

Step1: Insert an SD card into your device.

Step2: Go to Settings > Storage. Then Tap your SD card from the Portable Storage section. And select the three-dot menu in the upper right corner.

Step3: Tap Storage settings, then Tap Format as internal. And Tap Format SD card.

Step4: Tap Move content, then Tap Done. Your SD card will now function as internal storage.

Credit : https://www.lifewire.com/set-sd-card-as-default-android-6743109