How to insert bookmarks inside Google docs for Android without using desktop browser?
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.