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See Bitcoin Wiki. Above image illustrates a byte-map of transaction with each type of TxIn and TxOut.
We should attention at "Coinbase/Generation" (Coinbase means the first transaction in a block). Codebase's script is a arbitary data. Yep, it is random.
Therefore the signature of first transaction of a block is not used.
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related: What is the coinbase?, Where do bitcoins come from and what gives them their value?
– Murch – 2016-08-14T22:33:20.487