No, they are not.

Instances have websites but the bulk of the fediverse is done on a completely different layer, even a different port.

Fediverse instances are clusters of microservices. They usually include a database, a frontend and a backend. The backend is where the api is and where federation requests come in and go out. Thats where the magic happens.

If you want to test this, just disable the webserver (frontend) and watch the instance still working. You can also see this working when you look at the different frontends of some bigger lemmy instances for example.