Hi @SylvainLS,
Thank you very much for your answer, and my apologises for the delay.
So I removed the CustomParts folder on my operating system and checked if I could see some DUPLO parts in Studio. I couldn't see any references, including the DUPLO, Brick 2x2 (3437).
As a result, I extracted the ZIP file again containing all the DUPLO parts, changed all the names with a prefix (I used "ld"), and I was able to get the Brick 2x2!
I'm still not familiar with LDraw and its main differences with Studio.
I suspect LDraw is used to draw LEGO parts, and those can then be imported in Studio, am I correct?