Contacts

Set up iCloud on all your devices

Apps and features that use iCloud

Keep your contacts up to date with iCloud

you can use apple ID to gets contacts from your iPhone

use the same ID to log in your iMac

then open contacts app

after that, you can see all contacts in your apple ID

On the phone is Contacts turned on in Settings>Apple ID>iCloud?

On the Mac is Contacts turned on in System Preferences>Apple ID>iCloud?