Backend - what is needed and what is not?

Question

PHP

Last question for today.
In general, I assessed my strength, time, etc. + combed through several sites with code examples and, in general, with a description of a particular technology. At the moment, I'm most interested in the backend, and I have to get off C already (I'm learning just like that, for the sake of the base).
In general, for successful remote work in a year or two, and then freelancing, what do you need to know?
All I know now is nothing to do with the web.
The question is not about choosing a programming language, but about other technologies (and not even about a framework).
I kind of understand that you need to know how the browser and HTTP work, but this is not the whole list.
Actually, here's the question: what do you need to know (regardless of the language and framework)?

Answer 1

Apache / lighttpd + htaccess / NGINX
Protocols: HTTP, HTTPS, Sockets, RTMP
Tunneling , Proxy , OSI Model
HTTP headers , including Cookies
PHP / RoR , HTML, CSS , JS
long polling , AJAX, JSON - for JS
IRC ( semaphores, PCNTL , LOCK_EX, Mutex, Shared Memory ) - for PHP
Frameworks: Yii2/Laravel/CodeIgniter/Symfony/Composer - PHP, Angular/React+Redux/Vue/Node - JS
OOP, Design Patterns (MVC/MVVM/Singleton. ..)
SQL (MySQL + PHP , Search technologies (indexing, fetching)
Use: Linux, JSBin , Git / SVN, SSH, VNC, RDP

Answer 2

I can recommend a good review article on web development
tiendil.org/pages/webdev

Answer 3

And what difference does it make to the backender how http works and the browser, for the most part, except for web sockets?

Answer 4

Look towards vibed.org, a very promising tool for writing backends