[sudhirj]

Heroku runs your code on one set of colocated or near-located servers, usually behind a load balancer. There may be a database involved as well. You rent a VM for a certain amount of time and pay for that VM.

Fly Edge, Cloudflare Workers, AWS Lambda@Edge all take your code, deploy it atomically to 50+ datacenters located in almost every continent / developed-ish country, and run it in a "serverless" mode in response to HTTP requests on the closest edge to the request. They run your code in an on-demand container mesh that loads it up when there's actually a request for it, so you just pay for the total amount of time that your code runs.