I'm wondering where does a miner get an input for a transaction that spends the tx fees received from mining a block? It's not out of the coinbase transaction.
The transaction fees in a block are included in the coinbase transaction, which is a special type of transaction in that it does not have any inputs.
The value of the coinbase transaction’s output can be up to:
Current block reward + (sum of all inputs in the block - sum of all outputs in the block)
Claiming more than this will result in the block being invalid, but claiming less than this is allowed (though probably not in the miner’s best interests!)