How to use backend

Answer 1

Backend can vary according to chosen lang.
PHP:Laravel, codeigniter, yii...
Python:Django,Flask...
JavaScript: NodeJs+NextJs+Mongodb
C#: .Net...
Java: Spring...

And those not all

P.S.

anyway learn SQL if you wanna go to backend

Answer 2

From sololearn and the Tutorials on YouTube , Udemy, Udacity, Coursera , ....

Answer 3

you first learn backend, then you use it :)

Answer 4

How I learn from where

Answer 5

And How use this in solo for security