IVim

Boogaloo iVim is a text editor on iOS based on Vi IMproved by Bram Moolenaar.

Getting help on iVim

• Reddit, iVim HowTos.

Apple iCloud

All iCloud Documents are store in ~/Library/Mobile\ Documents/com~apple~CloudDocs. So:
Start by opening Terminal and then

$ cd ~/Library/Mobile\ Documents/com~apple~CloudDocs
$ ls -Al
.DS_Store
..
bash

• SuperUser, Access iCloud Drive via terminal.

Let us assume that on an iMac there is a file created in the subdirectory ./Documents/Text on the iCloud Drive named Example.txt.
The easiest way to open this file on an iPad.

• Open the App Files.
• Choose iCloud Drive under the tab Location in the left pane.
• Navigate to Documents and then Text.
• Now hold your finger on the filename Example and choose Share from the appearing list.
• On the 2nd row (with Airdrop, Messages, Mail...) go to the where More. Unless you see the App you wanna use for editing this file.
• From the appearing App list choose the required App. In this case that should be iVim.
• The file is now opened in iVim. Make some changes and write the file to disk using the vim command :w<enter>.
• Now iVim is showing where it save the result:
  /private/var/mobile/Library/Mobile Documents/com~apple~CloudDocs/Documents/Text/Example.txt

Where are my Files

Every App on an iPad has its own local storage. So you can not edit files that live in other Apps local storage.
Can you edit files in the iCloud?

Which files can be edited.