I don't want to appear rude, but OBS is a very complex piece of software. You have to learn its proper usage. It doesn't work if you don't even put a minimum amount of effort into trying to understand how it works. You also need to have a basic knowledge about video recording and processing.
You can find a huge amount of documentation here:
https://obsproject.com/wiki/
An overview of OBS' functions is found here:
https://obsproject.com/wiki/OBS-Studio-Overview
If you don't want to get a better understanding of what you do, if you want "just record something", OBS isn't the right tool for you.
If you just want a piece of Youtube saved locally, you might look for software that can be found by googling for "Youtube downloader" or "Video downloader" or something like this. Although Youtube frowns at doing this, I assume there are still much of these apps around.