I have a website based on angular and backend in C# + SQL server hosted on azure, and the website is a portal where any company can register with a main user on our system and can use our website features and services.

I want to use CRM for our system for sending emails, analytics and etc.

But don’t have clear idea how to use salesforce and how to integrate it with my existing system.
and along with that i have questions like can we use salesforce as complete backend for our system?
or we need to integrate it with backend by rest APIs to sync the user data with salesforce?

Please suggest and guide me how to start this integration part or i can completely use salesforce as backend APIs (if yes then how?)