firstpic

Above is a given table of data and I need to get the result data like the one below.

secondpic

SELECT ID
FROM
(
    SELECT ID, PARENT_ID
    FROM ECOLI_DATA
    WHERE PARENT_ID IN (SELECT ID FROM ECOLI_DATA WHERE PARENT_ID 
                        IN (SELECT ID FROM ECOLI_DATA WHERE PARENT_ID IS NULL))
) AS aa
ORDER BY 1;

I asked others to get the answer like the code above but I could not figure out the process of this code (subquery part). I figured out what was going on with the first subquery but from the second one, I was lost.
Can someone please explain detailed process? Thank you in advance.