How can I create sub note or any other alternative?

This is what I do.

Create a folder

In the folder create a note named “Index”

Open the index folder and add references to the Chapters. So, like this:

![[Chapter 1]]
![[Chapter 2]]

… and so on.

Initially, these notes do not exist. But, the Index note I can now see

Click the “Click to Create” button, and I now have a note named Chapter 1 that I can access via the Index note.

And so on.

The reason I use ![[Chapter 1]] is because I want to transclude the content of that note into the Index note. Just my personal tic. If you don’t want transclusion then use

[[Chapter 1]]
[[Chapter 2]]

There are other approaches, Templates, etc., but this one is quick for me.