Which template does content/aaa/index.md correspond to?

I have tried creating the following templates to no avail

layouts/aaa/index.html
layouts/aaa/single.html
layouts/aaa.html

According to the Hugo Lookup Order:

  1. layouts/aaa/single.html.html
  2. layouts/aaa/single.html
  3. layouts/_default/single.html.html
  4. layouts/_default/single.html

Btw, do you specify the layout in front matter?

This is a leaf bundle, not a section, so without layout or type in front matter the lookup order is:

  1. layouts/page/single.html (applies to all single pages in content root)
  2. layouts/_default/single.html

If you set type in front matter to about, the lookup order becomes:

  1. layouts/about/single.html
  2. layouts/_default/single.html

Thank you for your reply! I already know that index.md is just a regular page, and the template it uses is single.html or single.html.html

Now I find that after there is an index.md file in the aaa/ directory, if I add other articles such as: bbb.md, it will not be able to open. Is it true that as long as there is index.md, other articles cannot be added?

Yes. I just answered this question, again, in another thread:

Use an _index.md file to tell Hugo, “This directory is a section (branch bundle) that (probably) has member pages or subsections.” Use an index.md file to tell Hugo, “This is a just a page (leaf bundle), and it has no descendants.”

https://gohugo.io/content-management/page-bundles/