What are some of the important on page SEO tasks that you must do for best or better optimization and to help rank a page?

I would appreciate your validation on how relevant and important these tasks are for onpage optimization. And would love new ideas on this.

Top menu remains the same for all pages in all websites. It doesn't change at all. Side panel content or links are also mostly same in pages.

As per me, these do not contribute towards duplicate content.

The actual content body, what you write and explain is what needs to be considered as original, unique or duplicate.

The two possibilities here are:

The target keyword has low difficulty level for ranking: easy to rank; and They have many backlinks pointing with anchor texts suggesting the targeted keyword.

Did you properly check/read the page content? Aren't they talking about the keyword in some way. OR. They have mentioned location prominently.