Really sorry for asking such dumb questions so early on, since I got myself into this mess.
I found a guide on here about installing Plasma. I did that. Didn't like it. Tried to remove it.
I've used Synaptic to remove everything I can related to it, yet when I hit the login screen, it still shows the Plasma login and not the default Zorin one, which I now prefer.
How do I switch back over??
KDE uses SDDM, Gnome which is the default desktop for ZorinOS Core uses GDM and XFCE which is the default desktop for ZorinOS Lite uses LightDM. You learn more about Display Managers.
If you didn't remove gdm, you should be able to just
sudo dpkg-reconfigure gdm3
sudo systemctl stop sddm
sudo systemctl start gdm3
Perfect, thank you. When I got to the second command, screen went black and I had to hard reboot. But seems ok now!
Interesting! I use Devuan 4 and at point of install you can choose between SDDM or GDM - and I'm using Plasma - no issues! (Apart from the fact that GDM is heavily dependent upon SystemD which means my password that contains a hash cannot use the UK (en-GB) keyboard layout - I have to use the pound sign key for hash at login (but not with the lockscreen!)