How are transaction fees collected in Bitcoin? 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?

Thanks!
The coinbase transaction itself has the subsidy/coins for mine a block + all fees of the transactions of that block.