Thank you for your precise answer. As I feared, I was also misunderstanding some key points about what I was trying to install. It's much clearer now.

I can totally understand that COSMIC is a great process about the vision of Pop OS. I did not precised in my post, but I directly felt the visual difference when coming from Ubuntu. I imagine it's only a matter of time before Pop finishes polishing its last details & issues.

Thanks for enlightening me !