Sign in to the same Apple ID on both iPhone and Mac
On iPhone, go to Settings
Tap your name
Tap iCloud
Turn on Contacts
On Mac, open System Settings
Click your name
Click iCloud
Turn on Contacts
Open the Contacts app on Mac
Wait for iCloud sync to complete
Make sure both devices are connected to Wi-Fi
Make sure Contacts sync is enabled in iCloud on both devices
If needed, restart both devices
If needed, sign out of iCloud and sign back in on both devices