This app was mentioned in 8 comments, with an average of 1.00 upvote
I use this app to know storage.
https://play.google.com/store/apps/details?id=com.cls.partition
Avoid such bs apps. If you want your storage clean them clear cache from settings or use sd maid app.
https://play.google.com/store/apps/details?id=eu.thedarken.sdm
I don't trust default Xiaomi to help me know exacts so i use an app that give precise and precise details about storage and what is using how.
https://play.google.com/store/apps/details?id=com.cls.partition
I use this app to manage storage
https://play.google.com/store/apps/details?id=com.cls.partition
I use this app for knowing app and user files storage. Very worthy app.
https://play.google.com/store/apps/details?id=com.cls.partition
Clear cache. Use sd maid and do cleaning and reboot. Check storage.
https://play.google.com/store/apps/details?id=com.cls.partition
I use this app to find exacts.
https://play.google.com/store/apps/details?id=com.cls.partition
I didn't get what you want: an app that analyzes the internal partition or an app that can force-move stuff from the internal storage to the sd card?
https://play.google.com/store/apps/details?id=com.cls.partition
Latter case:
https://play.google.com/store/apps/details?id=com.buak.Link2SD
or https://play.google.com/store/apps/details?id=com.devasque.fmount
Read the instructions, at least Link2SD requires a specific partitioning.