"And is "earnings" defined as interest, dividend & capital gain?

Or does earnings include the net gain/loss in market valuation?"

Retirement accounts don't have these categories; it's ordinary income when you start to take distributions. Sheltered accounts have balances that grew or did not grow relative to Basis. You put in $5k; you take out $5k and the earnings from having that $5k in that account over that period.

The investment house or brokerage should return excess contributions and earnings.

If you really need to compute it yourself, use resources:

https://www08.wellsfargomedia.com/assets/pdf/personal/goals-retirement/taxes-and-retirement-planning...

https://www.investopedia.com/articles/retirement/04/042804.asp

https://www.thetaxadviser.com/issues/2020/apr/correcting-excess-contributions-iras.html

https://www.irahelp.com/forum-post/39031-calculation-earnings-excess-roth-ira-contributions-multiple...

https://www.fool.com/retirement/plans/roth-ira/excess-contribution/

"And yes, I'm also familiar with Pub 590-A and 590-B"

And Yes, use that, too. That's why it's provided.