Thanks for your help
I linked the wrong documentation, this is the part that made me think that the type of index.md is sets by default to about: https://gohugo.io/content-management/types/
Also I found an issue on Github, someone someone with similar thought:
https://github.com/gohugoio/hugo/issues/5714
The basic idea is to create the same folder structure as content inside the layout folder
Found another topic about this: here
Setting type in front matter seems the only solution