Any good links to learn the maths behind AES? I'm creating an academic poster for my EPQ on the mathematics of cryptography.  I would like to do a section on AES, however, I am struggling to find good resources to understand the maths.  I have watched the computerphile video on AES but it doesn't explain that much of the maths.  Is there any good resources out there that is able to explain the maths behind it in a not too complicated way so I am able to simplify the maths down so a sixth-former can understand it.
I think Lenstra's short document Rijndael for Algebraists is fantastic to get started on the maths behind AES:

https://math.berkeley.edu/~hwl/papers/rijndael0.pdf

You can find some information on finite fields on Wikipedia have a look at the example of $GF(4)$.

For a detailed overview of the cipher see the book  The Design of Rijndael:The AES Standard by its designers.

There is also The Stick Figure Guide to AES, at a more informal and descriptive level.