‘Bitcoins source code is open source, meaning it is being developed in a collaborative public manner. The code is available via the Bitcoin GitHub repository. If someone wants to make a change to Bitcoin, they can issue a pull request. The first step is somewhat of a design process, where developers are going back and forth with each other with their ideas and eventually, when there has been an agreement on a specific issue, developers come up with a final request known as a BIP (Bitcoin Improvement Proposal). This process has been in case since around 2011. The design proposal then goes to a lot of discussion and improvements and then the code gets written. Finally there will be a pull request on Github, which essentially is a a final proposal of what one wants to change. If it is a consensus change within bitcoin, meaning that it will change either the transaction rules, transaction states or Bitcoin values, manny will pay attention to it and check. A consensus change is a change to the agreement of rules that determinate which blocks and transactions are valid or not, agreement of which transactions have occurred, and agreement of how bitcoin has value, which effects if users accept bitcoin as sound money (for payment and as a store of value etc.) Then you get into testing, where people try to break the code. A lot of times pull request won’t make it, because there are concerns about them by the community. If a pull request does get accepted, it gets “merged into” the code. The persons that merge are known as “core maintainers”, there are several of those. It takes time and effort to become one. Usually people get nominated by others in the community, for long standing effort and contribution to Bitcoin. It then gets tested and reviewed again. Finally it gets released, people download it and run it. If users want the latest update, they need to upgrade. But there is no need to, as Bitcoin is backwards compatible, meaning that users that did not upgrade their version of the Bitcoin software, can still run a node.’
Explanation taking from the Podcast Episode: We Study Billionaires – The Investor’s Podcast Network; Episode BTC050: Bitcoin Policy, Fundamentals, &Impact w/ CJ Wilson & Jimmy Song (min 32).