For a given prefix, the interface will always pick the same identifier, (in fact, the eui-64 algorithm will pick the same identifier across multiple prefixes)
How does the algorithm typically work? Is there a loss of privacy since identifiers are reused across prefixes? If I replace my NIC or install a different OS on my machine will the address change?
There are other more privacy sensitive ways to generate obfuscated addresses.
But none of these matter, because RFC 4941 says a new random address is used for each request. If you surf the web on a server with a static address, it'll create hundreds of temporary, random addresses to make requests from. The server is reachable by the static address, but outgoing requests come from a random address.
I know, weird, right? Concept 2: IPv6 uses multiple addresses.
Yes, if you replace the NIC, the address will change. Different OS won't, if it uses EUI-64.