I got that every device has an unique global address. And with your explanation, I think I slowly understand the main difference between v4 and v6: v4 has limited addresses and therefor, the dyndns v4 has it´s own address on the router external. For v6, the routers dyndns client connects to the dyndns server - and the forwarding is done idependently for each device behind the router. That’s how my brain explains the situation.
The only missing thing is: Both my dyndns and my router do support both v4 and v6. As an earlier answer was that I client connecting to my net is deciding which version to take. Is there an easy answer on which basis this decision is taken? I may think of the possibilities of the client/device. As far as I know, mobile devices still rely on v4, so this will still be here for quite some years.
Anyhow, it would be nice if anyone could fill the holes in my brain concerning the subject.
Beside: I thank everyone to help me in understanding this. I really appreciate your help.