Sorry, for being too technical. And yes, that's should be the repsonse if you are going to do something that you never do before.

Should be more careful.
1 2 3. Documents or com.android.documentsui is one of the system app since Lollipo 5.0 above. It handles the Storage access framework for document picking, including the select SDCard prompt (similar to the Screenshot at Automagic external sdcard access) when you want access to write at external Sdcard, usb flashdisk or HDD. It can be found at setting > manage app > all > Documents. The location of manage app can be different in each phone, so need to locate it first (should be easy).
I am asking if it is disabled, because at xioami phones MIUI 7 above, this documentsui got disabled from beginning, causing the prompt Select SDcard not appear, no matter what we do. So need to enable it back first. Maybe by any chance, you disable this too, that's why I suggest to check if it is enabled.
4. Clearing this documents ui should reset all uri grant you have granted to all apps. Example if you use MiXplorer and ever granted it permission to write/create folder/file in microsd (via the select SDCard prompt), then after you clear data of Documents ui, the permission should be removed. You just need to re grant the permission again. Next time you wanna use MiXplorer to create folder at microsd, you have to go thru the same Select SDCard prompt to select the path to the microsd, and done. You have to redo this for microsd and all external OTG flashdisk if you write to them. So the risk is minimal, only need to regrant permission. No documents get deleted, despite the name. As long as you clear data only at this Documents (com.android.documentsui)
5. Of course you shouldn't root for just one purpose. I am asking if by any chance you already have root, that will be easier. Since I also haven't found a proven method to fix this write permission to sdcard. Have googled it since 2 years ago, still don't understand it completely. Sometimes it works, sometimes I have to delete the urigrant.xml to (that needs root).