how can i export my contacts from iPhone to mac with usb?

Using iTunes under the info tab you can select sync contacts. Apple temporarily removed this feature from iTunes so make sure you are up to date on your Mac.

It was in the early stages iOS 7 / Mavericks that it was removed but is back now.

You also must have something like iCloud contacts disabled, so the contacts must be on your phone not iCloud.

If something like iCloud contacts syncing is enabled iTunes won't let you do it.

If iCloud is enabled you can forgo USB altogether and sync online.

Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support

You can use iTunes to export contacts from iPhone to mac by following steps:

1. Connect your iPhone to your Mac using a USB cable.

2. Once iTunes has opened, press the iPhone icon that appears on the left-hand side of the iTunes window.

3. Click Info And then tick the box next to Sync Address Book and Contacts.

4. Now press Sync at the bottom of the screen and all your iPhone contacts will be copied to your Mac.

Or you can export contacts from iCloud, it's much more easier.

iCloud: Export contact information as a vCard