There are several different ways to configure IPv6 and the exact method depends on the network design and architecture of the business model.
Native and using IPv6 either on its own or in a dual-stack configuration alongside IPv4.
Address automatically obtained by DHCPv6 to an upstream server. Prefix delegation may also be used with DHCPv6 to deliver a routed subnet to a DHCPv6 client.
Stateless address autoconfiguration (SLAAC)
Automatically determines the IPv6 address by consulting router advertisement messages and generating an IP address inside a prefix. This is not very useful for a router, as there is no way to route a network for the “inside” of the firewall. It may be useful for appliance modes.
A method of tunneling IPv6 traffic inside IPv4. This is used by ISPs for rapid IPv6 deployment.
Similar to 6RD but with different mechanisms and limitations.
Not technically a direct WAN type, but commonly used. The customer builds an IPv4 GIF tunnel to a provider to tunnel IPv6 traffic.
While not technically a WAN type, IPv6 connectivity can also be arranged over a VPN such as IPsec, WireGuard, or OpenVPN. Most VPNs are capable of carrying IPv4 and IPv6 traffic simultaneously, so they can deliver IPv6 over IPv4, though with more overhead than a typical tunnel broker that uses GIF. These are good options for a company that has IPv6 at a data center or main office but not at a remote location.