The upstream network interface is configured to act as a DHCP client (both IPv4 and IPv6). It also sets this host as a router and will accept Router Announcements (RA)

#/etc/systemd/network/enp6s0.network
#
[Match]
Name=enp6s0

[Network]
DHCP=yes
IPv6AcceptRA=yes
IPForward=yes

[DHCP]
UseDNS=yes
UseDomains=yes
Anonymize=false
IAID=1
RouteMetric=100

This interface sends Router Announcements (RA) on the LAN with the 'Managed' bit set. The interface itself gets static IPv4 and IPv6 addresses. Hosts on the LAN will use SLAAC and configure addresses as 2001:db8:8073:19::/64 (Sent as an option in the RA - Line 'SubnetID=19')

#/etc/systemd/network/br0.network
#
[Match]
Name=br0

[Network]
Address=192.168.19.1/24
DNS=192.168.19.5
IPForward=yes
IPv6SendRA=yes
IPv6PrefixDelegation=yes
Address=fe80::19/64
Address=2001:db8:8073:19::1/64

[Route]
Gateway=192.168.19.20
Destination=192.168.21.0/24

[IPv6SendRA]
Managed=yes
RouterLifetimeSec=1800

[DHCPv6PrefixDelegation]
SubnetId=19

Here is only shown the parts that are relevant for prefix delagation. The DHCP sections are omitted.

#/etc/dhcp/dhcpd6.conf
#
...
# don't serve enp6s0 (WAN IF)
subnet6 2001:2db:8::8073/128 {
}

subnet6 2001:2db:8073:19::/64 {
    # Range for clients
    range6 2001:2db:8073:19::129 2001:2db:8073:19::254;

    # Range for clients requesting a temporary address
    range6 2001:2db:8073:19::/64 temporary;

    # Additional options
    option dhcp6.name-servers 2001:2db:8073:19::5;
   
    # Prefix range for delegation to sub-routers
    prefix6 2001:2db:8073:0100:: 2001:2db:8073:0f00:: /56;
}
...