I don't understand many things when I read source code of many popular websites. What should I do?

Question

I've basic skills of html, css, JavaScript, php & mysql. I've gone through sololearn, codecademy & coursera courses. I've also watched many tutorial videos. I've also created a form-filling page based on ajax php. After doing all this, I feel demotivated when I read source code of any popular websites.

Answer 1

The popular websites most of the time are very complicated. Don't feel demotivated. Code a simple website. Expand it's functionality little by little. Be patient.

Answer 2

They were done over several years by teams of developers.
And you are looking at the state of art.
There is only as much one person can do in his free time

Answer 3

One reason may be because the wep pages are not hand-coded.
Another may be because they use inline libraries.
Another because they use advanced js.

Whatever, you increase your skills, and you'll know . . .