Bitcoin is under constant development by a diverse (open source) team who operate quasi-independently. That is, those with a shared vision / idea form groups (development teams) in order to achieve that vision by writing code. If the team develops code that is accepted (merged) into the main branch of the project (Bitcoin Core, Bitcoin Unlimited etc) it can be said that ‘Bitcoin’ as a protocol has developed! Bitcoin isn’t a single implementation, by a single team. Rather, it is a protocol which has many different implementations. Crucially, these implementations must cooperate in order to avoid a fork in the network (with some users using Bitcoin A and others using Bitcoin B.
This process is clearly explained by the Core team on their Development Site and is well worth a read. The same procedure applies to all Bitcoin implementations.
Interested in seeing who wrote the code which forms the Bitcoin of today? Look no further than https://bitcoin.team/ (made by Andrew DeSantis) Below is a screenshot (17/07/2017) of the top 10 Bitcoin Developers (sorted by Rows which appear in the current revision!), clearly a congrats is in order to Pieter Wuille!
Clearly there is a long tail of development which has made Bitcoin possible, with 270 Developers having contributed code to the project! A log graph helps visualise this!
As does a Pie Chart of the same data! Incredibly 4 Developers have written more lines (which are in the current implementation) than all other developers combined! I expect that the number of developers required to account for >50% of the lines in the current implementation (who I like to call the 50%er’s) will increase going forward as more developers contribute and lines get updated. That said, it is incredible that so few have contributed so much to the Project! Long life Open Source!