When you backup your iPhone using a Mac, it overwrites the previous backup with the new one. Finder or iTunes does this to manage your Mac's storage and ensure only the latest backups with the newest data are saved and old ones are discarded. Overall, this sounds like a great thing, but there is a caveat.
iOS 15 public beta
Apple has released the first iOS 15 and iPadOS 15 public beta, which means that regular users can try out the new software. iOS 15 and iPadOS 15 comes with a bunch of new features and improvements, and with the public beta, everyone can experience it now.