If your ISP doesn't have NAT64 it will continue to use IPv4, and packets will continue to flow as normal. Apps however can't be IPv4 ONLY.
Ah okay. So the point of prefixing IPv6 with IPv4 is that if the ISP has only IPv4 switching, it will only grab the first 32 bits of the IPv6 address... which is the IPv4 address. So no ISP changes needed.
No, that is not how NAT64/DNS64 works...
You seem to be implying that every IPv4 packet will go through a carrier NAT64... but what if my ISP does not use NAT64? Will my iPhone send a regular IPv4 request, or will it prefix IPv6?