Could you explain each part of the code?

[Larz60+]

Expecting n = int(input()) to always return a number would be naive.
Someone will type text, so line 1 should be expanded to something like:

def get_num():
    n = None

    while(not isinstance(n, int)):
        n = input("Please enter a number: ")
        try:
            n = int(n)
        except ValueError:
            print(f"{n} is not numeric, try again")
    return n

n = get_num()

also, there is a builtin for line 2 in string:

# at top of script:
import string
# ...
alpha = string.ascii_lowercase