iOS 15.3 seeded to beta testers, WatchOS 8.4 beta goes to developers
Apple is just starting to see iOS 15.3 for public beta testers, so if you have a beta tester or developer account, you can download it right away. WatchOS 8.4, on the other hand, is only available to developers so far.
Both firmware updates focus on improving system stability and clearing some bugs from earlier versions of the software. There are no visible visual changes, nor are there any new features.
Keep in mind that beta versions of a given software are not intended to be used on daily drivers, as they may contain errors.