The direction React and Next.js have been moving is to give you very granular control over where your code runs, and when.

The previous model was that you simply have code that runs on a server when a request comes in, sends a response to the client, and then the code in that response is run on the client. Instead, now we have a situation where some bits run on the server, some of them on the client, which call out to some bits on the server again, and all of this can happen either before or after the server started sending the response. And then on e.g. Vercel, you also have edge functions as an additional permutation.

Which is kinda neat, but also massively complicates the mental model.