C . pool.ntp.org is indeed a virtual cluster of various timeservers. It provides a reliable and easy-to-use NTP (Network Time Protocol) service for millions of clients worldwide. The pool.ntp.org project allows systems to synchronize their clocks with internet time servers, which are part of a large virtual cluster1.
Reference:
pool.ntp.org: the internet cluster of ntp servers, which explains the purpose and functioning of the pool.ntp.org project.
How do I setup NTP to use the pool?, which provides instructions on how to use pool.ntp.org for time synchronization.
NTP pool - Wikipedia, which offers additional information about the NTP pool and its role in time synchronization across the internet.