I have read the documents on how to deploy Go and Python applications.
But most applications I’m working on recently deploy the frontend and backend as separated servers and make the frontend connect to the backend API for data.
Is this possible on fly.io?
You can deploy the frontend as a separate app, using something like GitHub - PierreZ/goStatic: A really small static web server for Docker.
You may also find Statics useful. This is a Fly feature allowing a single image to deliver static files and also run services. App Configuration (fly.toml), though with a full SPA you may need features statics don’t provide.