Is there documentation or base code for writing a Pulumi provider which is not backed by a Terraform provider? I’m looking into this for a customer of mine who wants to initialize their own product via their REST API. Yes, I know about

Dynamic Providers

but I would like to be able to compare both approaches.

There is a boilerplate https://github.com/pulumi/pulumi-provider-boilerplate

Is the grpc protocol between the host and a provider also defined in a file somewhere, like protobuf, so that a provider can be implemented in e.g. Typescript?

Yes - that's the one. FWIW - There is an implementation of the RPC interface in TypeScript as part of the "Node.js dynamic provider" here: https://github.com/pulumi/pulumi/blob/master/sdk/nodejs/cmd/dynamic-provider/index.ts