Splitting a string of numbers into separate cells: where to even begin?

With your number in A1
In B1 and copied to the right as far as you want to. Copy down as far as you need to also.
=IFERROR(MID(A1,COLUMNS($A$1:A1),1),"")

If you want them to be considered numbers (for calculations)
=IFERROR(MID(A1,COLUMNS($A$1:A1),1)+0,"")

COLUMNS($A$1:A1) is a way of iterating your value. it counts the number of columns between parenthese (in this case 1) as you drag/copy your formula to the right, it becomes
COLUMNS($A$1:B1) (which equals 2 columns) COLUMNS($A$1:C1) (or 3) and so on.