This function is very useful to recover automatically your contacts in case of loss or a phone change.
Open "Contacts" app :
- tap on ,
- tap on "Settings",
- tap on "Accounts" to check the account to synchonize (enter your Google credentials, if needed),
- tap on down the screen,
- tap on "Contacts sync settings",
- tap on "Manage settings", down the screen,
- enable "Automatically back up & sync device contacts".
NB : Your synchronized contacts are accessible with any connected device from Contact app or at contacts.google.com.
Synchronizing contacts from differents accounts at the same time is not possible.