When are mouse movements and key presses added to PureOS's /dev/urandom entropy?

Let’s say I throw a dice a few times and press 1, 2, 3, 4, 5 or 6 on my keyboard accordingly.

Under what exact circumstances would PureOS add these to the /dev/urandom entropy?

I’m pretty sure this is not a PureOS specific question. It is completely in the hands of the kernel, and I assume enabled by default. So, I don’t think there are circumstances. It just happens.
However, I assume you are aware that the kernel would neither add the numbers you typed nor the scancodes associated with those keys, but rather some inferred data. If your question is how that exactly happens, I’d say in short: very well (secure).
If you’re interested in the details, look at the code or search for articles on linux kernel entropy seeding.
Here’s a good start.