I will give some simple code to create Folder within Folder.
#1. Suppose we have a Folder Sections with items: Testimonials, list carousel, 3d carousel, gallery…
We need to create a folder within Testimonials, list carousel.

#2. Edit Site Footer

Add 2 Text Blocks

something like this

#3. Use this tool to find the ID of 2 Text Blocks.
In my example, we will have:
- Testimonials: #block-9db6127131ff6ca96f27
- List carousel: #block-d0631acca646a3cf9960

#4. Find the ID of 2 Folder Testimonials, list carousel in the Navigation Menu.
We will have
- Sections: div.header-nav-item:nth-child(6)
- Testimoinals: div.header-nav-item:nth-child(6) div.header-nav-folder-content>div:nth-child(1)
- List carousel: div.header-nav-item:nth-child(6) div.header-nav-folder-content>div:nth-child(2)

#5. Use this code to Code Injection Footer
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
// testimonials
$('#block-9db6127131ff6ca96f27').appendTo('div.header-nav-item:nth-child(6) div.header-nav-folder-content>div:nth-child(1)');
// list carousel
$('#block-d0631acca646a3cf9960').appendTo('div.header-nav-item:nth-child(6) div.header-nav-folder-content>div:nth-child(2)');
});
</script>
<style>
div.header-nav-item--folder .html-block {
position: absolute;
background-color: #3308f6;
right: -167px;
padding-left: 17px;
width: 150px;
top: 0px;
display: none;
}
div.header-nav-item--folder a:hover + .html-block {
display: block;
}</style>

#6. Result
When hovering on the Testimonials link.

When hover on list carousel
