I believe the structure comes from your book's TOC - or are you asking if the FM help is authored & published in FM?
I am asking what FrameMaker uses to create the table of contents in its responsive HTML5 output.
It uses the book_nameTOC.fm file that you create.
Your specific example looks like Fm's internal help system, which I do not believe is published via FrameMaker.
What you see in the screenshot is FrameMaker's Responsive HTML5 online help. It is produced with Adobe FrameMaker 2017 from the DITA Source.
The table of contents is produced in this example from the title elements of the DITA Source content (topics in a ditamap).
For unstructured FrameMaker files, the TOC is produced from the Book's TOC. The logic is explained in help topic "Format TOC for publish".