BIND9 can be configured in many different ways, mostly:
The DNS configuration files are stored in /etc/bind.
The primary configuration file is /etc/bind/named.conf, which in the layout provided by the package just includes these files.
NOTE: The root nameservers used to be described in the file /etc/bind/db.root.
This is now provided instead by the /usr/share/dns/root.hints file shipped with the dns-root-data package, and is referenced in the named.conf.default-zones configuration file mentioned above.
NOTE: BIND9 can be configured to be a caching name server, primary, and secondary at the same time: it all depends on the zones it is serving.
A server can be the Start of Authority (SOA) for one zone, while providing secondary service for another zone.
All the while providing caching services for hosts on the local LAN.