I solved the problem to sync between Mac and iphone thought iCloud drive.
Under Obsidian (on iCloud) create a vault folder, Vault1
create a link of Vault1 and move it to side by side of Obsidian Folder on iCloud drive.
On iphone, use the Obsidian Folder and it will consider Vault1 and a vault
On Mac, open the link of Vault1 as a vault.
Now both are linked.
If there is a way to open vault on IOS Obsidian, I believe someone could be able to use this way to sync through dropbox or other Clould driver , however, it need test.