array_diff in excel – finding missing values from column B that exist in column A

Fill column A with bigger sample space, aka the longer array.

Fill column B with smaller array, that has few or none of the elements from column A

Now, you need to find out values that are in column A but not in column B, here is what you do.
Create a separate column C and put this formula in cell C1 or $C$1

=IF(ISERROR(MATCH(A1, $B$1:$C$<LAST_ROWNUMBER_THAT_EXISTS_IN_COLUMN_B>, 0)), A1, "")

Ultimately your formula becomes match function dependable for whole column A. You just have to drag the cell C1 downwards.