And here’s how to add the same “A” record (named “www”) to each of the zones created above.

As you may have guessed this is the typical scenario where you’ve got to re-create some external zones, on the internal DNS servers. That’s needed in order for the internal hosts to reach some server with the “public” DNS name, but the private IP.
For the sake of completeness, let me also mention that you could achieve the same effect by leaving DNS as it is, and configuring “loopback NAT”/”double NAT” on the router/firewall. E.g.: an internal Host wants to reach an internal Server, given it’s public hostname, mapped to a public IP address. It asks the (possibly internal) DNS to translate the name. DNS doesn’t know the zone, it forwards the query to an external DNS Server, obtaining a public IP address that it hands back to the Client. Since its address is non-local, while trying to talk with the Server, the Client sends packets to its default gateway (possibly the router/firewall). The firewall matches the server’s public IP addresses, substituting it with the right private one. It also changes the source IP, swapping the Client’s with the firewall’s LAN address. This way Client and Server are actually communicating through the firewall, even if they’re both internal hosts. And the Server can’t tell Client A from Client B since every connection to it comes from the firewall’s IP address. That’s the main reason why I prefer duplicating the public DNS zones on internal DNS servers, with private IP addresses: you avoid routing internal traffic through the firewall, and avoid NAT where there shouldn’t be any.