Could the user include digits like 123 in the input characters?
Yes. They are characters just like any other.
As far as this program is concerned, a string of digits is a string of characters and is treated like any string of characters. The digits are NOT automatically converted into a numeric type. Here is another run of the program:
In a few pages you will see a program that reads in a string of digits and converts them into a numeric data type. Then arithmetic can be done with that data.
Is data from the keyboard always only characters?