Their router needs to know what devices are behind it so it can route to them. But if by "their router" you meant "your router"... your ISP doesn't need to know at all. They send all traffic for your prefix to your router, and your router figures it out from there. Your ISP has no idea what devices are involved.
The existence of privacy addresses suggests that some thought was put into this.
There are no more "your routers" for about 10 years. The router is given to you by the ISP, and you don't have the root password for it, you can only reboot it over an APP on your phone.
The prefix is also not delegated to that router, the router does npd proxying, and the ISP only routes the ips which have the corresponding NAs recorded into its database.
That's... not how things work in general. It would be possible for an ISP to do that, and I'm sure somebody somewhere does, but they could do the exact same thing on v4 so you don't get to blame v6 for it.
If your ISP runs the default router for your own LAN, they'll have full visibility into it on both v4 and v6. That's just how IP works.
Most ISP I have seen implement it like this. A large chunk of those "most" also require you to bind a phone number to each separate hwaddr appearing in the network via SMS. (Not all though.)
Those few that implement it differently, do the following:
They serve ULAs to the customers over slaac, and nat6 all the ULAs to a single ipv6 assigned to the router (actually a wifi hotspot).
I totally believe that where you live things are done differently, but this is exactly why ipv6 critics call it defective. It allows too large a variety in implementations.
That's the adaptation layer between Ethernet and IP.
Broadcast was renamed to "all nodes multicast". ARP was renamed to Neighbor Discovery.
Slight improvements: ND isn't broadcast, but multicast based on several bits of the IP address. This allows NICs to filter most of the irrelevant ones based on multicast MAC address. And subnet broadcast addresses were removed. There's only local broadcast to your own subnet and not to someone else's subnet, since IPv4 routers found that to be a bad idea and mostly started blocking it anyway.
No it's not. Slaac and NA make it a totally different beast.
If ipv6 only had dhcp6-pd, it would have been "just like ipv4 with longer addresses".