How does bitcoins github work?

i just don't understand how the https://github.com/bitcoin/bitcoin really works. who owns it? who makes changes to it? if there's a set of people making changes to it doesn't that mean bitcoin is really not decentralized? (i know bitcoin IS decentralized, that's why i know i'm missing something here.) i noticed there was a small amount of contributors to it, less than 50. what is this for and who decides the changes? also how is bitcoin updated for example when segwit was added and how taproot is going to be added. if bitcoin is peer to peer how do you just "update" it?