OK, lightdm is your default display manager. I had to reconfigure my system to bring it up and recall how it works. When you get the initial login screen, go to the upper right corner, and you will see a line of options, small print. The leftmost icon looks like an SD card with a wrench on it. Click there, and you can select the desktop environment you want. It's currently set to the default, which you probably do not want. You should be able to select either XFCE or Plasma, and perhaps also either with X11 or Wayland. Click on the one you want, make sure the correct button is selected, and enter your username and password. That should start a session using your choice. You can configure that however you want. You can choose a different session at each login if you want, and this is useful when you're trying to decide which you prefer. The default login should be whatever you used last, but it may be set to a single session type. You can always choose at login.