why isn't my MacBook Pro syncing with my iPhone 12

Aren you signed into Settings/Your Name and System Preferences/iCloud (Apple ID for Catalina or later) with the same Apple ID? Is iCloud syncing turned on for both the computer and the phone?

Try going to iCloud.com using a computer browser to see if the items are there and whether they are from the device or the computer. That should show whether the device or the computer isn't syncing. If the computer, try going to System Preferences/iCloud (Apple ID if you are using Catalina or later) and stop syncing. Wait a couple of minutes, then recheck syncing. If the device, go to Settings/Your Name/iCloud and do the above. As a last resort, you can also try signing out of iCloud and then sign in again.