Noctalia is a desktop shell for window managers. Tiling and scrolling window managers (such as i3, MangoWM, Niri, etc.) are only window managers. They don't have any kind of graphical way to launch applications, there's no dock, bars or widgets of any kind. They only manage the windows, and you need to provide the rest by installing a number of different applications. Noctalia is a shell that provides a full desktop experience with all the bells and whistles you might expect from full desktop environments like KDE and Gnome.
The previous versions of Noctalia were based on Quickshell which was a memory hog. It used few hundred megabytes of memory per each display you had. Noctalia v5 is a complete rewrite in C++, which especially on multi-monitor systems uses only a fraction of the memory usage compared to Noctalia v4.
I switched from KDE to Niri+Noctalia a few months ago. It's highly customizable and pleasing looking environment. There are templates for different applications which applies your chosen color theme to applications so that you have unified look across all (or most) of your applications.
And to be clear, Niri is my daily driver, and I could probably figure out how those details work under the hood, but I don't really have the drive to do that right now.
I love Niri, but it takes a lot of intentional configuration!