How to programmatically organize Saas (cloud service)?

Question

Software design

I plan to develop a kind of small online CRM system.
Saas system, completely cloud-based, without downloading programs to your computer and other things.
Quite a few systems already operate on this structure.
And I wanted to know how it is accepted or necessary to develop such systems in terms of programming??
For example, Bitrix24, when creating an account, a subdomain is created on their website, and there you are your own owner and build your business structures, your customers and employees.
So, how are such systems programmed?
For each client, a client is created with its own folder with its files, a separate database, and so on.
And in this folder are the copied php files for the functionality and style and design files.
But if later I update the functionality, then how can I update these php and service design files for all users?
Or a database is created for each, but the executable php and design code is in one place and all users use it.
Explain in detail please.
Maybe there is some common culture of creating such projects that I don't know about?
How to create such a service? So that naturally you can then update and supplement the functionality of the service. And these changes have become immediately for everyone.

Answer 1

Or a database is created for each, but the executable php and design code is in one place and all users use it.

Exactly like this and nothing else.