Where can I find the structure and layout of what exactly gets hashed by miners? I understand that a miner has a list of bitcoin transactions since the last time a a block was discovered. The first entry is the reward for who discovered the previous block. As more transactions gets added, they're appended to this list. This part is speculation, but I think there's also a nonce that the miners inject in order to produce a hash output that meets the difficulty.

Where can I find out the specifics about what exactly is hashed? I'm looking to understand the exact bytes and structure.
The data that's hashed is the block header. You can find a description of the format and layout at https://en.bitcoin.it/wiki/Block_hashing_algorithm