I'm looking into the structure of blocks in the Bitcoin blockchain, and I can't figure out when\how the fees collected by miner of the blocks are transferred to the miner. According to what I can tell they are not a part of the coinbase transaction (which as I understand has to be constant since changing it after discovering a block would change the hash of the block). Are the fees paid as separate a transaction? Are they written directly to the UTXO without appearing on the blockchain? Something else?