Matrix, fragments children widgets id

It's the fragments' props property you need to override to get what you want:

  "file": "track.json",
  "props": {"variables": {"n": #{$} }}

what is the widget chain here? Is it like this: root -> matrix -> fragment (track.json) -> panel -> all track widgets

Yes, the fragment widget is a wrapper, just like the clone widget.

Is there some way how to check it (i tried to show it using web developer tools in ff, but didn't find it)?

The html structure should represent the widget structure, each element with the widget class that has a data-widget attribute is linked to a widget instance.