Notebook 7 is now based on Jupyter Server, which is a new server application that allows to run multiple Jupyter applications (e.g. Notebook, JupyterLab, NBClassic, etc.) on the same server.
This means that Notebook 7 is able to reuse many of the existing server extensions from the Jupyter ecosystem as is.
The Jupyter Server documentation provides a guide for migrating from the classic notebook server to Jupyter Server
The Jupyter Server documentation provides a guide for authoring server extensions