Universal link must be configured as root domain in Xcode Associated Domains?
I searched for a long time, and read the whole apple document, but i can not find the obvious answer or definition about whether we must config the Associcated Domains as root domains, eg. applinks:apple.com, applinks:www.apple.com or we can config it as: applinks:www.apple.com/path/to/some/site ?
You can't find an 'obvious' answer to that question, because simply needing to ask it indicates you misunderstood the document you were reading (or didn't in fact read the whole thing) 😉.
The applinks: entries must be root domains. All that does is inform iOS that when your app is installed, it should go check that root domain for an apple-app-site-association file. If one is found, that contains the mapping information for URLs (such as /path/to/some/site).
Here is a full set up guide: https://blog.branch.io/how-to-setup-universal-links-to-deep-link-on-apple-ios-9/