SEO : specific structure for a website vs an app

I’m working for a infotainment website. They have a website but didn’t do any SEO work from now. They want to restructure the website. I have always heard that e SEO website have to get a specific structure extremely organized : home > categories > subcategories > articles, is it right ? A colleague told me : look at the YouTube website. It doesn’t seem to have this very organized structure; it has a lot of differents tags for différents videos. It doesn’t seem so organized. I didn’t know what to answer her… Can you please help me with your expertise and tell me what a website structure must be for the SEO ?

I was also wondering if the website and the app have to have the same structure. I think it can be extremely different but I wanted to be sure.

Thank you very much in advance for your help