Is it a norm in android apps to only have bottom navigation only on parent activities

I have checked multiple apps, and in most of them, bottom navigation is only visible on parent activities, when i go in any inner activity bottom navigation goes away, so my question is that is there any guideline regarding this available in material design, or everyone is just following this approach without any proper guideline,

and is it a right approach to have bottom navigation only on parent activities.

and in which cases we hide navigation on child activities, if not in all.