For quarto related question, you could also ask directly in
Developer team would be able to answer directly.
I believe they are called index.qmd because they will be converted to index.html which will then allow the url youblog.com/blog/post1 to work. See for example Quarto blog article Quarto - Positioning Content in the Margin and source code quarto-web/docs/blog/posts/2022-02-17-advanced-layout at main · quarto-dev/quarto-web · GitHub
So this is why the convention, and why it is made this way.
What is the use case behind not having this index.qmd file but another name ?