How is it possible that I'm able to use plasma when I only installed lubuntu?

I just installed lubuntu dual boot with windows 10, it runs smoothly on my low end laptop. I'm confused though, when I turn on the computer, I get 2 choices Ubuntu (not lubuntu) or windows. And when I choose Ubuntu, and the login screen is displayed I get to choose either lubuntu or plasma. What's confusing me is that I just realized that KDE plasma is a desktop environment for kubuntu. Since it's very customizable I'm sticking with it over lubuntu. I just want know is plasma installed automatically with lubuntu installation? Or did it just install kubuntu. (apologies for the noob question here I'm just getting started with Linux and so far it's better than ram hole windows10)

this is the output

Reading package lists... Done Building dependency tree
Reading state information... Done neofetch is already the newest version (7.0.0-1). neofetch set to manually installed. 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

KDE Plasma is not installed with a Lubuntu install. KDE Plasma uses the identical Qt5 as Lubuntu's LXQt uses, however KDE Plasma also requires KF5 which Lubuntu does not come with (though parts (some libraries) of it are included as some programs, eg. KDE Partition Manager, require it).

To have the Kubuntu/Plasma option - you'll have caused it to be installed due to a depends rule as a consequence of something you've installed or by your install method.

What is included with Lubuntu can be found at

• https://packages.ubuntu.com/focal/lubuntu-desktop

and what's found on the 20.04.3 ISO is

• https://cdimage.ubuntu.com/lubuntu/releases/20.04/release/lubuntu-20.04.3-desktop-amd64.manifest

You mentioned using the "replace method" of installation, and whilst I don't know what you mean by that exactly; a "Install using existing partition" (a Lubuntu Quality Assurance checklist install type) will cause all manually installed packages that existed on the system prior to install, to be re-installed after your fresh Lubuntu is installed.

ie. if you added kubuntu-desktop to your system prior to the install of Lubuntu (and Lubuntu was installed using any option that didn't format you / partition), then those manually installed or added packages that existed in the old install, will be re-installed (using internet if available) after your new Lubuntu system is installed (from your installation media).

In my linked Lubuntu checklist testcase I used the example of including clementine as an example as it's a music player I like (and not included by default with Lubuntu installs), but it applies to any packages on the system if available in Ubuntu repositories.