The documentation indicates the pre-defined NTP servers are asia.pool.ntp.org and in.pool.ntp.org. My observed behavior is that the pre-defined server, at least after having previously enabled an NTP server, is pool.ntp.org.
See pool.ntp.org: The NTP Pool for vendors
What is the ntpclient and how does it work? Does it blindly sync with the first responding server?
Use Custom NTP Server
Specify the NTP server IPv4 Address or IPv6 Address or domain name to synchronize time with it. If custom NTP server is defined, time is synchronized with custom server and not with pre-defined servers.
Devices use NTP Version 3 (RFC 1305). One can configure up to 10 NTP servers. At the time of synchronization, it queries each configured NTP server sequentially. When the query to the first server is not successful, device queries second server and so on until it gets a valid reply from one of the NTP servers configured.