They they don't need NAT that is simply called routing. The ISP sends every packet that is in your assigned /64 to your routers IP address. It's called prefix delegation [0]
Yes they get a /32 by default (at least in RIPE) larger allocations need justification. But there are 2^32 /64 subnets in a /32 so every ISP gets a complete IPv4 internet of /64 they can assign to their customers at will. Your devices assigns itself a random IP address from that /64 network your ISP gave you via prefix delegation.