Hi there. I will tell you what I learned and what my assumptions are. Perhaps I can learn some more 
First, the "assumptions part"...
DHCPv6:
- is a "legacy" technology
- is missing "default route" information
SLAAC (aka RA aka Router Advertisement):
- is the new kid native to IPv6 you should rely on
DHCPv6/SLAAC:
-
I can use none, both or one of them.
-
The IP address the interface of my device gets from either of them is always something like <prefix>:<address>/64 or <prefix>:<address>/128
Meaning: <prefix>:<address>/56 does NEVER exist as an interface address, as the subnet is part of the <prefix> and therefore the address is displayed with "/64"
So if I configure some local subnets, the prefix differs but each subnet is "handing out" /64 addresses.
Now my example with Starlink...
A) There is router advertisement and radvdump says:
#
# radvd configuration generated by radvdump 2.19
# based on Router Advertisement from fe80::200:5eff:fe00:101
# received by interface bridge
#
interface bridge
{
AdvSendAdvert on;
# Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
AdvManagedFlag off;
AdvOtherConfigFlag on;
AdvReachableTime 0;
AdvRetransTimer 0;
AdvCurHopLimit 64;
AdvDefaultLifetime 300;
AdvHomeAgentFlag off;
AdvDefaultPreference medium;
AdvSourceLLAddress on;
AdvLinkMTU 1500;
prefix 2a0d:3344:1500:73fd::/64
{
AdvValidLifetime 300;
AdvPreferredLifetime 150;
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr off;
}; # End of prefix definition
RDNSS 2606:4700:4700::1111 2001:4860:4860::8888
{
AdvRDNSSLifetime 360;
}; # End of RDNSS definition
}; # End of interface definition
B) There is DHCPv6 and dhcp6dump.phar (work in progess) says:
Eth Src: 00:00:5e:00:01:01
Eth Dst: ff:ff:ff:ff:ff:ff
IPv6 Src: 2a0d:3344:1500:73fd::1
IPv6 Dst: fe80::ee08:6bff:fe56:bc57
Message type: 7 (REPLY)
Transaction ID: 0x3380D1
Option: 1 (CLIENTID)
Type: 2 (DUID-EN)
Enterprise Number: 43793 (systemd)
FE CE BA B8 (....)
Option: 2 (SERVERID)
Type: 2 (DUID-EN)
Enterprise Number: 50960 (TODO)
00 (.)
Option: 25 (IA_PD)
IAID: 3980038939
T1: 300s
T1: 480s
Option: 26 (IAPREFIX)
TODO
00 00 02 58 00 00 04 B0 38 2A 0D 33 44 15 DE 79 (...X....8*.3D..y)
00 00 00 00 00 00 00 00 00 (......... )
Option: 23 (DNS_SERVERS)
2606:4700:4700::1111
2001:4860:4860::8888
What I did... (with systemd-networkd)
A) I configured IPv6 with DHCPv6 only:
[...]
[Network]
DHCP=ipv6
DHCPPrefixDelegation=true
[...]
I configure a token for fun:
[...]
[DHCPPrefixDelegation]
Token=::1234:1234:1234:1234
[...]
Then I get (via "ip ad")
[...]
inet6 2a0d:3344:15de:7900:1234:1234:1234:1234/64 metric 256 scope global dynamic mngtmpaddr
valid_lft 1192sec preferred_lft 592sec
[...]
Additionally I can also configure a subnetid for even more fun:
[...]
[DHCPPrefixDelegation]
Token=::1234:1234:1234:1234
SubnetId=42
[...]
Then "ip ad" says:
[...]
inet6 2a0d:3344:15de:7942:1234:1234:1234:1234/64 metric 256 scope global dynamic mngtmpaddr
valid_lft 1158sec preferred_lft 558sec
[...]
See that little "42" that is in place of "00"? Nice.
Also, "journalctl -fau systemd-networkd" says:
[...] systemd-networkd[6951]: bridge: DHCP: received delegated prefix 2a0d:3344:15de:7900::/56
[...] systemd-networkd[6951]: bridge: DHCP-PD address 2a0d:3344:15de:7942:1234:1234:1234:1234/64 (valid for 19min 59s, preferred for 9min 59s)
That's a ::/56 from Starlink! 
I also had to configure the default route by hand:
[...]
[Route]
Gateway=fe80::200:5eff:fe00:101
GatewayOnLink=true
[...]
(I stole the info from SLAAC/radvdump!!!)
B) Now I configure IPv6 with SLAAC only (with token again 
):
[...]
[Network]
IPv6AcceptRA=true
[IPv6AcceptRA]
Token=::1234:1234:1234:1234
[...]
And "ip ad" says:
[...]
inet6 2a0d:3344:1500:73fd::1234:1234:1234:1234/64 scope global dynamic mngtmpaddr noprefixroute
valid_lft 267sec preferred_lft 117sec
[...]
So as you can see, they got slightly different prefixes for DHCPv6 vs. SLAAC (See "[...]de:79[...]"-vs-"[...]00:73[...]"-part.) Also the subnetid with SLAAC in my case (?) is "fd". I don't know... could this be default, like "00" with DHCPv6. ("fd" is kind of reminding me of the "fd::"-style local addresses.)
So, my remaining Questions are:
- How can I see, what kind of prefix my ISP hands out with SLAAC? /64? /56?
- If not for the output of journalctl how would I even know I had /56 with DHCPv6?
- If I had e.g. /56 with SLAAC like with DHCPv6, how would I set SubnetId?
- Is SLAAC capable of non-/64 advertisement at all?
- Why would an ISP use both: SLAAC and DHCPv6?
- Have I misunderstood subnets with SLAAC?
Anyway, you may think, my post contains too much useless information. Maybe... But a post like that would have helped me earlier. So, just comment and correct my assumptions or answer my questions if you like...
Thanks everybody!