As per https://spotlightjs.com/sidecar/npx/ running the sidecar:
If you start the sidecar independently, you can visit the overlay at http://localhost:8969/ and use a built in overlay to view your application.

However, when navigating to http://localhost:8969/ I am getting a 404 with no content.

I am running a react-native app so can not launch spotlight through the web frontend and am on windows so the bundled MacOS electron app is not available.