In the left column: how was the numerical part of the answer found (addition, subtraction )

You can find the numerical part of the answer because it is the same on the right side.

For example:
 ¹      << Regrouped
  89
+
  67

146   <<=== I had to press space so I could position it.

So that also happens in subtraction, the left side can be found by doing the same steps in the right column.
For example:

67
-                    <<=== this time there was no regrouping
12

55

The mode of the numbers is 29.