Is what happened with the video calls that involved nudity and arousal considered Zina (fornication/adultery) or masturbation?

What you did is uglier than mere masturbation; it is the fornication of the eyes. As stated in the Hadith: "The son of Adam has his share of fornication written for him, which he will inevitably attain. The fornication of the eyes is looking..." However, this is not the fornication that necessitates the prescribed punishment (حد). As long as you have repented to Allah, strive to fulfill the conditions of repentance and remain steadfast upon it.