What's the downloaded_rom folder and what's its difference from the Download folder? I have a Redmi phone with Android Nougat 7.1. In the file explorer I found the downloaded_rom folder, next to the Download folder. The first one is empty. What's its purpose?
To view, both Download and Downloaded_Rom folders, you may need to have file manager applications such as google's files go, es file explorer, default file manger(for mi devices -> Mi file manger/Explorer) on devices. 

Download 


Folder created on internal storage of android device by default. This folder is one among the other internal storage default folders such as Android, DCIM, media, Movies, Music, Pictures, Ringtones etc
It is used to store files that are downloaded from internet using mobile browser apps like chrome, firefox, and default browsers of each manufacturers.
Some devices provides Download access through Downloads app which is placed on app launcher. (Eg: Pixel, Xiaomi etc)


Downloaded_Rom


This folder is created on internal storage, which is commonly seen in xiaomi/Mi devices.
This folder contains the OTA update zip file(Stock ROM), which is downloaded using Updater application in Mi devices. stock ROM is the OS of the device which is released by the device manufacturer(Xiaomi).
Note: After, completing updation, the zip file will be automatically get cleared. So, this folder may be empty, until next OTA update notification and started downloading. 
If you download OTA/update zip from xiaomi's website which is opened using browser application, the file is available on Download folder. Copy and paste zip file, to this folder and you can install this update through updater app.