There are many different players/clients, and they have their own versions that aren’t tied to the server or other client versions. As far as the web app is concerned, 4.121.1 is the latest available, but that’s only if you’re accessing the web app through app.plex.tv. When you navigate to the web app using your IP:port, you’re accessing the web app that’s bundled with the server, which is usually a few versions behind app.plex.tv. The server release notes will mention when the bundled web app is updated (e.g. the latest beta, 1.40.0 bumped it to 4.118.0, but 4.116.1 is the latest version available in the public release).
