Where to does macOS's iTunes save iPhone's back data?
On macOS your backups are in /Users/your username/Library/Application Support/MobileSync/Backup.


Open Finder, and select the “Go” menu at the top of the screen. Hold the Option key, and select the “Library” menu option from the Go menu. (On Lion and above, Library will only appear with the Option key held.)
From Library, navigate to Application Support -> MobileSync -> Backup.


Source (from a blog post I wrote on the topic): https://deciphertools.com/blog/where-are-iphone-backups/