That answer is still unknown to mankind.
Yes there are many thousands
The smallest organism on Earth is a nanobe
No, Earth is not considered an organism. It is a planet that supports various forms of life but is not a living organism itself.
there are many, like earth worms and cave dwelling fish
The second organism that appeared on Earth was likely a simple single-celled organism, possibly a prokaryote such as a bacterium. This would have followed the appearance of the first organism, which was also likely a simple organism.
How do dead organism return inorganic back to the earth
I assume you meant single "celled" organism. It is an organism (life form) that consists of only a sigle cell as opposed to most all other lifeforms on earth which are comprised of many (billions) of cells.
We call such past organism "extinct".
Bacteria
Yes
the rock
because of the gravity of the earth