Bitcoin is a totally open source protocol, and its development is therefore collaborative. Anyone can copy or modify it.
Of course, each update must be agreed upon within the network.
Bitcoin Improvment Proposals
The first version of One Bitcoin a Day source code was of course provided by Satoshi Nakamoto, and the famous cryptographer Hal Finney actively participated in its early development.
Bitcoin improvement proposals are called Bitcoin Improvement Proposals or BIP. They can be viewed on the Bitcoin GitHub repository. This was formalized by Amir Taaki on August 19, 2011 and accepted by the network. It is BIP 0001.
The BIPs must be precisely structured. They must clearly explain the technical specifications of the improvement. Once submitted to the community, their status will vary as follows:
Bitcoin Improvment Proposal – BIP – Life Cycle
Diagram describing the life cycle of a BIP (BitcoinWiki)
A long and demanding process
The „draft“ of the modification is submitted to the network. The developers will then rotate and return the code in all directions in order to check that it does not pose a security flaw. It can be rejected or referred. If the proposal is accepted, the final code will then be integrated into the existing one. A new version of the protocol is then proposed to the network nodes.
In order to be finally integrated into the Bitcoin source code, and active, each modification will then have to gather the consensus of the community. That is to say that the network nodes will have to update their client software with the new version of the code, including this modification.
This process promotes caution and maintains the security of the entire network. As a result, it can take several years to implement a change.
While everyone is free to contribute, some developers are more prolific than others. For example, Luke Junior has proposed no less than 213 commits on Github! There are currently 248 contributors to the Bitcoin source code.