Procedural materials questions, sites ,stock ,tutorials

I’ve been learning procedural materials by working youtube tutorials.
Any recommendations for tutorials. ?

Where are the Stock procedural materials that come with Blender ?

I’d like to downloads some procedural materials from safe sites from the web .
I don’t know anything about Blender sites.
Any recommendations for good n safe sites ?

Are there any Blender sites that I should stay a way from ?

Thanks a lot