How to insert bookmarks inside Google docs for Android without using desktop browser? I need the functionality of inserting bookmarks within the Google drive or Google Docs Android app. I can do that from within the desktop browser.  Is this feature already there, and I am just not seeing it?

This is not the same thing as adding a shortcut to the Android devices home screen as indicated by: Can I make a Google Doc shared with me more accessible on my Android device?

Google is a large company. When it creates apps for Android, it needs to be on parody with the desktop browser unless there are specific technical reasons why that's not possible, and sometimes it is not possible, but not in this particular situation for sure.
It is (still) not possible currently to insert a bookmark with Google Docs for Android (version 1.20.494.04.33). The only option to insert a bookmark is via the computer as stated on Docs Editor Help: Work with links, bookmarks, section breaks, or page breaks:

Add a bookmark
Bookmarks are not available on Android; you’ll have to use a computer.