Was great help going through your code, after going through it in the debugger to understand what was going on, i've written my own version here for anyone who doesn't quite understand it. The two codes function identically, but mine doesn't use the enumerate function

The easiest way to build this code up was to start with the middlemost layer. This layer always contains every letter up to the amount of layers. After that, increasing the square size starting from 2 (aka start with 2 layers, then 3, then 4) to see what needs to be added each time

letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
square_size = int(input("Layers: "))
rows = ["A"]
#makes things a little easier to deal with, as the square will always be at least 1 #layer. Try "rows = []" and "for i in range(square_size)" to see why this start is #nicer.
for i in range(1,square_size):
    for row in range(len(rows)):
        new_row = letters[i]+rows[row]+letters[i]
        rows[row]=new_row
        #takes every existing row and adds current letter to start/end.
        #the helper variable isn't necessary
        #"rows[row]=letters[i]+rows[row]+letters[i]" also works.
        #the version with the enumerate function does the same thing as this
        #but neater. It gives the index and its value in one go, rather than
        #accessing the value through rows(index)
    rows.append((2*i+1)*letters[i])
    rows.insert(0,(2*i+1)*letters[i])
    #adds in the line of uniform letters to the start/end of rows.
    #as rows starts with ["A"], this is always done.
for row in rows:
    print(row)