Mar 17, 2009
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Your Mac's Specs
2018 15" MBP, 2019 11" iPad Pro, iPhone 11 Pro
Whenever you sync your iPhone to your MB, iTunes first creates a backup automatically.
You'll need to open iTunes with your iPhone connected (wired or wirelessly if you have iOS 5.x). From the main screen you can select to have your backups on your Mac or iCloud.

If by "back up" you actually mean "sync" contacts to your MB, then on the main screen for your phone in iTunes go to the Info tab and select what exactly you want synced from your phone to your MB and click on "Sync" at the bottom right.

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