I'm not quite understand, and getting confused, if as per your saying
1. To save our phone contact locally only, then we should uncheck the icloud contact sync ? (Setting > contact > account > icloud > uncheck)
To save only locally, you have to uncheck contact sync for all accounts. Doing so will not upload your contacts to any account.
2. And that's mean, if we set our default contact to icloud, no contacts were saved in our phone at all ?
Enabling contact sync for iCloud will upload all your contacts to iCloud. You are not able to save them locally. However, all contacts will appear on your phone, even if you're offline.
3. If we have icloud and google account checked sync (as per my previous thread setting), and we choose icloud as our default account (setting > contact> default account> icloud), that's mean the contact will only sync to "icloud" and not other account contact (although we set "green" as well) , even locally ?
Having multiple accounts enabled for contacts is confusing. You can select in the Contacts app what group you want to have shown and say you chose for showing only Google contacts, then new contacts will be saved to google, not local and not to iCloud, regardles of what you have set as default account.
This is the main reason for me to stick with iCloud only. Whenever I will have the need to save my contacts to Google, I can export the contacts on iCloud.com and import them in Google.
4. If our previous setting icloud as our default account, and we uncheck icloud & google accounts' contact, then our phone won't have any contact showed ?
Like I said in point 3, you can select where you want to save new contacts by just showing that group. Sometimes some contacts get saved locally and not to iCloud nor Google.
You can check this yourself by going to the contacts app and deselect all groups and select just the local. This will show you all contacts saved locally.
5. If that's correct, then how to let icloud sync the contact locally to our phone
By only enabling contacts for iCloud, all contacts in iCloud will show in your Contacts app. Even though they're not saved locally, they will appear and stay there until you disable contacts for iCloud.
6. And how to backup/sync contact to google account as well (from icloud)
Thx
Go to icloud.com and then to the contacts tab. Select all contacts and click the settings wheel, you will see there an option to export all contacts to .CSV which you can import in google/contacts.