To use Micro SD card as Internal Storage (Enable Adoptable Storage) on a MIUI 11 phone, follow this tutorial:
https://miui.blog/any-devices/steps-enable-adoptable-storage-miui-11-phones/
or https://c.mi.com/thread-2601107-1-0.html
Following this tutorial you will be able to:
Use Micro SD Card as Internal Storage on MIUI 11
Enable Adoptable Storage on a MIUI 11