I’ve only started looking into this, but my impression is that a
Zettelkastan is both a thing (a big slip box of interconnected notes, either
on paper or in digital form) and a methodology (literature notes, fleeting
notes, etc).
Something like Roam Research provides the slip box part, and you can
therefore use it as a Zettelkastan, but the app also lets you use it as a
personal wiki, possibly alongside the Zettelkastan functionality, if one
wants to draw a distinction between the two.
I’ve decided I want to try building up a Zettelkasten (I’ve settled on
org-roam), but I think a personal wiki would be useful too, and I’m unclear
if I should be keeping the two things apart.