The bodies are specified in a schematised way, so all of this code can be generated. This is not hypothetical - Pulumi’s Kubernetes support, Azure (“nextgen”) support and GCP Native providers are all built like this.
It’s quite possible that the provided schema may need some augmentation for some resources (for example, semantic diff for IAM policies) to get the best plan-time experience, but this is a different world from having to hand write schema and CRUD logic for every resource type per the last 7 years of Terraform AWS support.