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

Match]
Name=enp6s0

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

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

[IPv6PrefixDelegation]
Managed=yes

[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=900

[DHCPv6PrefixDelegation]
SubnetId=19