To better explain what result I need, I made a text block below as a pseudo sheet to illustrate.

• Col A (source) is the manually entered text data. Col B and C (desired result) will have formulas so that cells from A are populated in them as per the pattern.

• Pattern for B is: fixed number of cells from A in its order, followed by delimiter "HELLO" then same number of following cells from A again and so on. Pattern for C is: Text "SET" plus a counter of number of delimited cell sets in B. (Please note that A will have more (or fewer) number of filled cells (added/deleted occasionally), so B & C must adapt accordingly.)

• I've been playing around with formula
  =arrayformula( if ( mod( row(A:A), 6 ) = 0, "HELLO", A:A) ) for B but it skips every sixth row in A, and also I can't figure out how to do C at all 🙁

       (source)     (  desired result  )

        A           B           C
1       one         one         SET 1
2       two         two
3       three       three
4       four        four
5       five        five
6       six         HELLO
7       seven       six         SET 2
8       eight       seven
9       nine        eight
10      ten         nine
11      eleven      ten
12      twelve      HELLO
13      thirteen    eleven      SET 3
14      fourteen    twelve
15      fifteen     thirteen
16      sixteen     fourteen
17      seventeen   fifteen
18      eighteen    HELLO
19      nineteen    sixteen     SET 4
20      twenty      seventeen
21      twentyone   eighteen
22      twentytwo   nineteen
23                  twenty
24                  HELLO
25                  twentyone   SET 5
26                  twentytwo
27
28
29
...     ...         ...         ...