Syncing contacts across devices - best practice

Take help of a web browser and enter in www.icloud.com with the apple id and password used in an iPhone , the contacts that were saved in will be visible select the desired contacts and click on gear icon in the left bottom corner and click on export as v card .

Open finder , click on downloads , right click on .vcf file and open with contacts .app( default ) .

The address book will get opened , a pop up window will appear stating are you sure you want to add number of cards , click on add .

Again open your address book the contacts that were in the iPhone will be transferred under the section “ All contacts “ in the Mac address book .

Sign out from www.icloud.com used with Apple ID and password for an iPhone .

Then enter in www.icloud.com with the apple id and password used in the Mac , click on gear icon and click on import as v card all the contacts will be transferred in the iCloud server , click on create group on the bottom right corner an untitled folder will be created in the side bar and the contacts from iPhone are all transferred to the iCloud server account .

See some articles iCloud: Export a contact as a vCard from iCloud Contacts

https://support.apple.com/kb/ph3605?locale=en_US

https://support.apple.com/kb/ph2667?locale=en_US

See this old thread https://discussions.apple.com/thread/250414800?answerId=250796057022#250796057022

Have you tried this:

Keep your contacts up to date on all of your devices with iCloud

https://support.apple.com/en-ie/HT205754

The reason why you have not all contacts in iCloud it's maybe related to the fact that you have contacts also from other accounts (gmail, outlook, facebook, etc.)

The simplest method is to sign all devices into the same iCloud account with the same AppleID and then enable "contacts" in the iCloud settings. Your contacts will then sync via iCloud across all your connected devices.

However, if you're using different services on different devices (e.g. sync'ing with Gmail, yahoo, MS exchange or some other email service that offers contact sync) you may want to instead ensure all devices are using that same account and service instead, if it holds your "master" contacts list.

Welcome !