Hopefully this helps give a better idea of how it works under the hood: https://nitric.io/docs/reference/providers/custom/building-c...

Not just a feature of Pulumi, its just used as the IaC of choice for the out of the box deployment providers. This can be replaced with anything.

At its core Nitric is really a tool to collect infrastructure requirements from an application and those requirements are delivered to a server that is built using the Nitric deployment APIs.