How does Bitcoin receive upgrades if it is decentralized?

Title mostly says it all. I was talking about bitcoin with a friend of mine about the decentralization side of things but I thought about the upgrades/updates bitcoin receives every now and then. Like how did the hard fork with BCH work and how is this upcoming Taproot upgrade going to roll out? I thought Satoshi dipped a while ago on BTC. Is there another team that has been developing on bitcoin? Any explanation would be really helpful I can't really find an answer online.