All it's doing is allocating address space. And 64 bits means you have an astronomical amount of that. It only needs to become actual memory when you use it. A page at a time.
No, you can typically only assume 48 bits of virtual address space; the kernel takes half of it, so really only 47 bits. Newer CPUs provide 57 bits (e.g., Zen 4) or 52 bits (certain ARM64 models). And that's provided that the kernel plays ball; apparently some Android configurations only support 39 bits of user virtual address space.