Not sure I fully understood your question. My answer above is only about development.
In a deployment situation it doesn't matter what server you use to serve the client part, it's just static HTML, JS, CSS
The Dart server application can serve these files as well a for example Nginx while the backend is still a Dart application.
I also don't see how splitting client and server into 3 Dart projects (client, server, shared) is related to any of this.
Splitting it has the advantage thouhg, that additional dependencies for client-only and server-only don't interfere.
It caused me quite some headaches that some packages I needed only on the client were incompatible with the newest versions of packages I only need on the server and therefore couldn't update to the latest version or had to use `dependency_overrides?.