The general rules for addressing in IPv6 remain the same as the way you would calculate IP addressing in IPv4. The difference is that an address has 128 bits, of which the first 64 bits are the prefix, consisting of different parts and the last 64 bits are the interface ID.
Addresses can be assigned manually, through stateless autoconfiguration by reading information from router advertisements or through DHCPv6. A combination of stateless autoconfiguration and DHCPv6 is possible. The client learns from the router advertisement whether he has to contact a DHCP server.