Is there a basic script / code for email marketing?

I want to develop a platform for email marketing and autoresponders such as Aweber, Mailchimp and the like (but much more basic).

I researched 'email marketing' on google and found TENNIS of platforms and sites that offer this service.

So I thought there should be a base / script code that companies pick up, modify, customize, and then sell the service.

The problem is that I do not know how to search for it, what words to use or the name of that type of code ... so where do I find it?