A company has started releasing iOS 15.3 for public beta testers, so if you have a beta or developer account, you can download it right away. On the other hand, WatchOS 8.4 update is only available to developers.
Both updates focus on improving system stability and removing some bugs from previous versions of the system. There are no obvious visible changes and no new features.
Keep in mind that beta versions are not intended to be used on everyday operating systems as they may contain bugs.