A coinbase transaction is the first transaction in each block. The coinbase transaction distributes the block subsidy, which is currently 3.125 BTC per block, and also collects the cumulative fees of all transactions in the block. Since this transaction is creating new bitcoin, the coinbase transaction is valid without any inputs. To see an example, check out this past coinbase transaction in Block #650,000 on Blockstream: there will be no inputs, and the single output will be of the amount 6.25 BTC plus the 0.244131 BTC of fees the miner collected.
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