How is the Bitcoin Mined in Genesis Block Brought into Transaction? I'm studying the source code Bitcoin, and have no clue to this question.

It looks like the genesis block is mined "out-of-band" (given a timestamp, and a message with no transaction information) and then hard coded into the source code before compiled (with the nonce, hash, Merkle Tree Root hash, and a modified timestamp if no nonce found with the original one). After Googling, it is said that 50 BTC was mined in the genesis block, is the 50 set by the Bitcoin protocol? And will the "50 BTC" message participates in the mining of the genesis block? (No from my current understanding). If the "50BTC" not part of the message for genesis block generation, we're free to change it to, e.g 500BTC after mining the genesis block, and I think that will not violate the original Bitcoin protocol. With this question pending, I am not sure how the first Bitcoin is created (looks out-of-band with the generation of genesis block).

Any hints will be highly appreciated!
is the 50 set by the Bitcoin protocol?


Yes, the block reward started at 50 BTC per block, and halves roughly every 4 years (every 210,000 blocks to be precise).


  And will the "50 BTC" message participates in the mining of the genesis block? (No from my current understanding). 


Yes it is, the 50 BTC is in the output of the coinbase transaction from that first block:
https://blockchain.info/tx/4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b

The first transaction in any block is the coinbase transaction and pays the block reward to the miner. And because the hash of the transaction is part of the block header which gets mined, it does directly contribute to the hash of the header.


  If the "50BTC" not part of the message for genesis block generation, we're free to change it to, e.g 500BTC after mining the genesis block, and I think that will not violate the original Bitcoin protocol. 


1) it is part of the "message" for the genesis block, so changing it would change the genesis block which would make it rejected by every other node on the network.
2) The block reward of 50 BTC is enforced by all nodes so changing it would also make the block invalid too. 


  With this question pending, I am not sure how the first Bitcoin is created (looks out-of-band with the generation of genesis block).


The 50 btc from that genesis block are the first 50 BTC ever produced (although they cannot be spent, see this question for why: Why can’t the genesis block coinbase be spent?)