Website with a program running in the background

I think this is a basic question, but I never faced this situation before so I don't have a background on this. Honestly, I don't know how to search for it either.

Basic example: If I have a website showing a new prime number every 10 minutes ( 2 3 7 ...) and people can visit www.primenum.io and see it.

So I have my express server running on Heroku, how do I keep doing the math for the prime numbers in the background? In another server and communicate through a WebSocket?

That's my question. Any guide will be appreciated, even how to search for it on the internet.