who is managing/controlling/guiding Bitcoin development?
The bitcoin users, and anybody who wants to, just run the bitcoin software that makes you a node, and you are part of the people managing and guding the bitcoin development, there is no leader or single human being that is in control, it is all collective stuff and that is why it is decentralized.
Is there any team or a person behind all the decisions being made?
Bitcoin is an open source software and anybody can join in the decision making through running the full bitcoin software, those who do that are called nodes, and they alongside the miners communicate to validate transactions and reject the ones that are against the bitcoin protocol. So it goes without saying that there is no hidden or special team like in centralized services, anybody can be involved if they want to.
I guess the code of Bitcoin was changed since Satoshi disappeared?
In centralized services one person or a small group can make a change, in bitcoin that is decentralized a change can't be made in that way or forced on the network, devs can implement additions to make the network better and more user-friendly like Lightning network implementation for example, but they can't make a change to the existing protocol because it won't be agreed upon by the entire community.