Every major OS uses the IPv6 privacy extensions by default. Short version: your device frequently picks a new address at random.

a new address from a prefix that still identifies you? or a new address from a mixed pool of customers of your ISP?

A new address from the network that your ISP routes to your house. That's basically identical to the way an ISP routes a single IPv4 address to your house.

Generally you'd get a network prefix from your ISP of 56 to 60 bits, your router would handle deciding how to make a network out of the remaining 4 to 8 bits of the network part, and then devices would automatically assign themselves unique addresses with the remaining 64 bits for the host part. One thing to note is that devices can and almost always do have multiple IPv6 addresses. Commonly: one for a local only address (like a 10. or 176.16., 192.168. address would be for IPv4), one based on the MAC address of a device, and now one (or a few) that is randomly chosen and periodically cycled to prevent some of the tracking issues you mention. The problem overall though is that the network part of the IPv6 addresses would still be the same and would be similar in tracking capability for a customer/household as how an IPv4 address is with NAT.