The simplest server part for your site, how to implement it?

Question

Backend

I learn web development by implementing projects from my head.
Created a website for an online store, where the layout is through Bootstrap, CSS styles and Javascript.
Implemented frontend:
Each button shows the initially hidden container .and hides the rest, through JQuery , show() and hide() methods , the user can select one of two products, read the page with information and reviews, without reloading, by the way, is it a primitive Single Page Application? (SPA)
It just so happened that the project kicked off my subscriptions, and I really wanted to sell products through my simple creation. And now it's time to use it with the backend, and from the backend I need:
1. Storing goods (DB) and obtaining their quantity (for information on the client side)
2. Automatic sending of emails upon purchase, and so on.
In general, the simplest server-side, for the simplest site ...
Where to dig? Is it possible to do with pure JS for such requests / responses from the database?

Which server-side JS + framework to choose? I'm familiar with OOP, in principle it doesn't matter what kind of YaP, (only not PHP !!! ! ! ), just to understand the server side as best and easier as possible for a crustacean in this matter . Django seemed to be a rather complicated framework, and Python is still sugar, but the server side somehow avalanches there, and it’s hard to digest everything ...

Answer 1

https://php-start.com/course/php-start-practice

Answer 2

flask
django is not complex