INSERTSTRING

INSERTSTRING(original_text, text_to_insert, position)
It inserted a substring into an existing string at a given character position:
It was essentially a more flexible version of Excel's REPLACE or LEFT/RIGHT text-splicing pattern.

=INSERTSTRING("abcdef","XYZ",4) = abcXYZdef

Just like Excel's MID, the first character is position 1.
If position = 1 it puts at the start
=INSERTSTRING("world","hello ",1) = "hello world"

If position is greater than the string length it puts at the end
=INSERTSTRING("abc","XYZ",99) = "abcXYZ"

empty original then it returns just the inserted string
empty insert then it returns original unchanged

=LEFT(A1, pos-1) & insert & MID(A1, pos, 9999)