Non-Consecutive Cell Referencing

May I be a bit harsh.  In my opinion you are beginning your adventure of learning Excel by focussing upon obsolete techniques, maybe forced upon you by the use of legacy software.  In these days of dynamic arrays, the concept of relative referencing an dragging formulas down is over-complicated and confusing!

If you wish to reference every 5th element of an 'array', one way of doing it is to wrap the array across 5 cells at a time.

wrapped
= WRAPROWS(array, 5)

You will see the references you require in the first column.  To select the column one simply chooses it

every5th
= CHOOSECOLS(wrapped, 1)

These are most useful in situations in which the other columns are used in separate calculations.  If they are not used, INDEX will work

= INDEX(array, SEQUENCE(n,,, 5))

Putting these together, either by nesting or by using the LET function to define variables

every5th
=  CHOOSECOLS(WRAPROWS(array, 5),1)

or (a bit more advanced)

every5th
= LET(
    wrapped, WRAPROWS(array, 5),
    CHOOSECOLS(wrapped, 1)
  )

every5th
= LET(
    k, SEQUENCE(n,,,5)),
    INDEX(array, k)
  )

I am not trying to get at you.  It is simple that I believe that you should be encouraged to start your journey by travelling forwards, not back to the past.

Maybe the other expert contributors to this discussion have a different opinion.