How providers work: plugins that call APIs, downloaded by `terraform init` from the Terraform Registry

A provider is a separate program, usually written in Go using HashiCorp's plugin framework, that knows how to manage one platform. Terraform core starts the provider process when it needs it and talks to it over a local RPC protocol. The provider receives requests such as validate this resource configuration, plan this change, create this object or read this object's current state, and turns each one into calls to the platform's API, using the credentials and settings in its provider configuration.