like how to make one without Icon-o-matic (or other theoretical Icon software)? Is there a description of how to write an HVIF by hand in a text editor?
HVIf is a binary tightly packed format, optimized for disk space, you won’t be able to easily write it in any text editor
There’s a blog post that explains a lot about the technical details of HVIF:
http://blog.leahhanson.us/post/recursecenter2016/haiku_icons.html
This might be interesting if you want to build a own editor or library for the format,but nephele is right,writing those files by hand is almost impossible and not what the format was designed for.