It can be hosted by either WebAssembly or by the server (in which case the server will render the DOM and send diffs to the client over a WebSocket connection). Blazor Server probably isn't the best choice for a popular SaaS app, mainly because of its dependency on WebSocket connections and less-than-perfect reconnection logic.
I'm optimistic about the auto-rendering mode, which will serve the app via Blazor Server (using a WebSocket connection) the first time the user hits the app. It will download the WebAssembly DLLs in the background so that the next time the user comes by, they will get the WebAssembly-hosted version. It's an interesting mix combining the best of both worlds (hopefully).
I'm optimistic about the auto-rendering mode, which will serve the app via Blazor Server (using a WebSocket connection) the first time the user hits the app. It will download the WebAssembly DLLs in the background so that the next time the user comes by, they will get the WebAssembly-hosted version. It's an interesting mix combining the best of both worlds (hopefully).