so like.. not to be too cynical but how does he know his representation is correct or at least not misleading? a lot of youtuber content is just made up.

Yeah... It's dangerous to accept random YouTubers' content as fact mindlessly.

I'm not sure all he says are correct, but at least he uses the target companies' engineer blogs, external articles, and some open-sourced part of systems (and list them in the video's detail section). His main targets are often big techs like Twitter, Uber, and Netflix, so I guess such documents are often available.

Narendra's content is awesome, but I think you're right to be skeptical. His content is more focused on how to answer system design interview questions about how the companies operate.