Turn this array into a matrix Take a non-nested array as input. Turn it into a matrix by using the following method:

Let's say my array is [1, 2, 3, 4, 5]

First, I repeat that array 5 times: (the length)

[[1, 2, 3, 4, 5],
 [1, 2, 3, 4, 5],
 [1, 2, 3, 4, 5],
 [1, 2, 3, 4, 5],
 [1, 2, 3, 4, 5]]


Then, I read it along the diagonals:

[[1],
 [2, 1],
 [3, 2, 1],
 [4, 3, 2, 1],
 [5, 4, 3, 2, 1],
 [5, 4, 3, 2],
 [5, 4, 3],
 [5, 4],
 [5]]


I flatten this array and split it into pieces of five (the length):

[[1, 2, 1, 3, 2],
 [1, 4, 3, 2, 1],
 [5, 4, 3, 2, 1],
 [5, 4, 3, 2, 5],
 [4, 3, 5, 4, 5]]


This is code golf. Fewest bytes wins.
Jelly, 11 bytes

WẋLŒDUṙLFsL


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Explanation

               Input: array z
WẋL            length(z) copies of z
   ŒD          Diagonals (starting with main diagonal)
     U         Reverse each
      ṙL       Rotate left length(z) places
               (now the top-left diagonal is in front)
        F      Flatten
         sL    Split into chunks of size length(z)