Web Development - How to create multiple instances of a website for different users and with different settings?

I am working on a web project and I am trying to see if I can use something already existing instead of creating it on my own (if not, I would still look for suggestions on how to pull this off).

Basically, I am creating the structure of the platform and I would like to know what is the best way to create multiple instances of the same website for different/selected users and with different settings.

For instance, I want the company 'A' to use this website. They would like the background to have their colours and logo, the language to be English and to allow the users Smith, Black and Brown to access/manage it.

At the same time, company 'B' wants the same website but with different background, in Japanese and for all his 100 employees.

How do I create multiple instances of my website for each of these companies and, perhaps, being able to assign "companyA.example.com" to company A and so forth. Is there a framework already supporting this?

If not, what would be the best way to do this?

The idea is to have it scalable for as many users and instances as needed.