2 of 2 added 28 characters in body

Python 3, 105 102 bytes

3 bytes thanks to Mr. Xcoder.

def f(a):l=len(a)-1;r=range(l+1);return[[a[i][j]+a[l-j][i]+a[l-i][l-j]+a[j][l-i]for j in r]for i in r]

Try it online!