Mounting an extra partition as a fake external SD card

Nowadays, phones don't have SD cards anymore, but some have a ton of storage. So, I was thinking of creating a fake external SD card using an extra partition to be able to store all of your zip flashable files, pictures, and music and have it stay when you inevitably format user data when flashing a ROM or whatever. My question is: **How to properly mount a partition as an SD card so that Android thinks you have an external SD?** (you see it in a file manager, can store pictures on it directly, etc.) What I've done already : - I have created an extra partition on my OnePlus 6 with parted, it's sda17 - I have successfully mounted this partition to be able to use it normally (mount -t ext4 -o rw /dev/block/sda17 /external_sd) This command allows you to navigate to /external_sd and see your files, but Android doesn't think that you have an external SD card. I'm thinking of using a Magisk module to mount everything properly at every boot (if this is even possible)