Who decided the rules for Cryptocurrency in the first place

Question

When a miner solves a block, he gets a block reward made out of thin air. For Bitcoin this is currently around 12.5 BTC. What dictates this specific amount of money? Is the the locally ran software? If so, can't that be tampered with? Does the miner ask other clients what the current block reward amount is? If so how does it know it's being fed the right updated information?

Same goes for the number of zeroes found on the hash. If a miner finds a hash value like 00000000000000000000000000000000000000000000000000000000010101111110110101010101 he would then check how many zeroes it starts with. Let's say the current solve requires 30 zeroes. Who makes that rule? How is it updated? At what points does it change from 30 -> 31? Who makes that decision to increase or decrease it. What if one computer thinks it's 29 and not 30. What stops people from gaming the system?

Same with block sizes. What stops miners from sending blocks with increased maximum sizes? Would clients reject the block if they don't match a certain size? If so, how do they know what are the maximum amount of transactions? Who told them?

Answer 1

A single miner can tamper with a block as much as they want, changing block award or difficulty or double-spending, but such a block will not be accepted by the rest of the network. This implies that the rules of how the network will operate are decided upon by the nodes in the network.

Bitcoin network needs to reach a Consensus to accept a specific block. As long as more than half of the nodes of the network are "good" ones, the tampered block will be rejected. As the consensus mechanism will always favor the majority of the participants in the network.

Now you ask who is responsible for deciding this. Well, this functionality is implemented by the Blockchain P2P Network.

The consensus mechanism makes sure that the nodes in the network 'play nicely'. If there's a malicious node that tries to tamper with the block by validating false transactions, he would require to spend a huge amount of computational power. And the rest of the nodes in the network will reject his block during verification. So, his efforts, energy and resources all go to waste. So it's only logical for all the nodes in the network to play by the rules. Also, the nodes which validate transactions and create blocks are incentivized to stick to the rules in order to keep the network running and receiving the benefits of being a part of the network