> I have no idea what stage manager is and I don’t wanna look it up.

It’s just yet another way to manage windows. It’s fully optional, I never even remember it exists. It was introduced in iPadOS 16 / macOS 13.

https://support.apple.com/guide/ipad/ipad1240f36f/

https://support.apple.com/guide/mac-help/mchl534ba392/