Ask HN: How to build web apps that deploy user environments?

Think apps along the lines of web flow, squarespace etc. Whereby when a new user joins they have an environment to develop in and direct a custom URL to if needed.

I'm agnostic about whether they'll be on shared resources or single servers, but was wondering if there's any frameworks/systems that will help with this type of architecture? Even reading materials on how to get started? I confess I'm not even sure which terms to Google...

I'm willing to swallow cost if there's a SaaS that makes it easy, for example.