Any good links to learn the maths behind AES?
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.