As far as i remember reading before it is even a layer on top of screen or tmux and you could even choose which.
Never tried it though. Tmux does me fine (I used screen for decades but these days it shows its age)
That's something else. Zellij is a reimplementation in Rust. Tmux is perfectly fine as well but for me personally Zellij is more approachable if I want fix a bug or add a feature.
You are thinking of byobu. Zellij is an independent replacement for tmux.