Depending on the situation, the data lives either within the browser or within the OS. Chrome releases ship versions of tzdata that correspond to the version of tzdata shipped with the ICU it uses, and they do backport updates to prior Chrome releases within a certain window. Apple has a sideband way of deploying tzdata to all devices that doesn't appear via the normal Software Update mechanism. So it all depends on which particular OS/browser combo you're interested in and the decisions those owners made.