Can someone explain the relationship between “decentralized” cryptos/ tokens and the organizations responsible for them?

For example I use the Brave browser. BAT is supposed to be a decentralized token for its ad platform. But if the token is truly decentralized why is it that the Brave team only has access to it (maintaining and upgrading it) and other platform aren’t able to freely implement it?

I also have a Helium miner and it’s a similar thing. Only the Helium team has control over it and they approve who is allowed to build miners for its blockchain.

Sorry if this is a noob question but I’m genuinely confused here. Can someone lease clarify?