You can do whatever you want in your code.

... but, we could solve it using a module like numpy, I guess?

We have nothing to judge if what you say is true, and if there were actually an issue, nobody could fix it. We don´t even know which language you're using. Please read this before posting issues: https://docs.codewars.com/training/troubleshooting/

Not at all (and I can't remember a single kata where it would be the case).

(Python) are we supposed/required to use numpy arrays for this one?

See https://en.wikipedia.org/wiki/Node_(computer_science).

As for "why don't we speak 'Node' much when using python", I would suggest u read a similar question on so, it answered my similar concern.

Search for single linked lists.

(Python) Could someone point me to a source explaining exactly the thing this kata calls "nodes"?

Honestly, I have no idea what these "nodes" even are, Spyder doesn't recognize the "Node()" function as a name,
There's this tutorial:
https://www.tutorialspoint.com/python/python_nodes.htm
which suggests it's just when you make a class yourself and put in attributes that point to other attributes, in a very general sense.
Searching on this site: https://docs.python.org/3/search.html?q=nodes doesn't give me any results that seem to be what I'm looking for.
And so on...

Really, it seems I can only find talk about "nodes" in python that mean very different things depending on context and nothing like "The Python Nodes" which is what this kata seems to take for granted.