What Is the Role of the Block Header in This Initial PoW Phase?

The block header is a crucial data structure that contains metadata about the block, including the previous block's hash, a timestamp, the Merkle root of the transactions (though empty in the PoA template), and the nonce. The PoW process involves repeatedly changing the nonce until the resulting block header hash meets the target difficulty.

It serves as the input for the hashing function.