KDE is a community (this year it turns 30!) and Kdenlive is part of it. Visit the website and read more about it.

Regarding you Qt question, there is the KDE Free Qt Foundation, more info: https://kde.org/community/whatiskde/kdefreeqtfoundation/

I cannot tell you which DE to choose, I guess try them both and use what you like.

KDE distros that work well, try Arch (and derivatives like CachyOS), Fedora and there is also KDE Linux (but that is still alpha)