Parsing YouTube?

Question

YouTube

Hello!!
I need to parse a video from YouTube, or rather get the correct url of any video on YouTube...
Now I'm at the stage where everything is good and bad...
https://vkmanager.com/youtube_dowload.php?vid=L_dW...
Here is my parser...
Videos are parsed, but not in
This is the script that parses everything...
se...

Those videos that cannot extract links, these links are available

and as a result of some manipulations and decoding, you can get a completely normal link, but in the browser it does not work - error 403....
How to be?
How do YouTube video download services work then?
help me please