Where you type in your password to log in, look for a cog (
) icon of some sort or a dropdown tab that says Plasma.
XFCE has this:
And LightDM, which pretty sure also comes with KDE, has a “Sessions menu”:
That “Sessions menu” is what I’m referring to and is a little Whisker icon next to my name.
Example screenshot I found for KDE online:
It would be that little tab at the bottom left.