I am on the latest LineageOS, and "df -h" does give me a lot of information, along with a reasonable figure for the device size under /storage/emulated.

I am using a ConnectBot local session to see this.

When I run this as root, the output of df is different, but it is not immediately obvious why.

If you can see the subdirectories, "du -sm *" will recurse and report usage by directory.