Function resolve_transaction 

pub fn resolve_transaction<DB: Database, Tx, T>(
    tx: RpcTxReq<T::Network>,
    default_gas_limit: u64,
    block_base_fee_per_gas: u64,
    chain_id: u64,
    db: &mut DB,
    tx_resp_builder: &T,
) -> Result<Recovered<Tx>, EthApiError>
where
    DB::Error: Into<EthApiError>,
    T: RpcConvert<Primitives: NodePrimitives<SignedTx = Tx>>,

Goes over the list of TransactionRequests and populates missing fields trying to resolve them into primitive transactions.

This will set the defaults as defined in https://github.com/ethereum/execution-apis/blob/e56d3208789259d0b09fa68e9d8594aa4d73c725/docs/ethsimulatev1-notes.md#default-values-for-transactions