It reuses the runtime you already ship

After the outlet is swapped, the freshly inserted islands need to hydrate. The router calls the same re-hydration walk the streaming primitive already uses.

New islands light up; the shell is left alone. The router package is tiny because the heavy lifting already lives in the runtime.

The like button and "time on page" timer below are outlet islands — they are re-created on every navigation, and torn down when you leave.