The following are not-properly-researched theories I made up
I think x came from "unknown". Perhaps at one point people used a cross (x) to represent something unknown. The symbol may have extended to mean "variable". Then math advanced and people begin making graphs with variables and the logical next variable is y.
abc are used when these variables run out, because they are intuitive to pick.
ijk in programming loops came from "index". The reason we use "index" and not "count" or "tally" is because in early computer days, someone decided to use "index". For example, your browser still automatically fetches "index.html" if a path doesn't return a valid response.
ijk in vectors may have come from quaternions, which in turn came from complex/imaginary numbers. Since i represents "imaginary", that could have been carried over to vectors.
The xyz orientation in 3D program is purely a choice by the developer. It is which ever orientation they are comfortable with developing.
y is up because of it's commonly used for graphs, but y is down for computer graphics. The top-left corner of a screen is (0, 0) and as the position moves downwards, y increases.
We do use Greek letters in other coordinate systems! For example, a spherical coordinate system often uses (r, θ, φ). I think these symbols may have already had connections with other aspects of math (like θ is commonly used for angles). They are not used generally to avoid confusion.
You already said China so I guess I'll go Hong Kong.
Hong Kong (香港) means fragrant harbour. The origin of this name is unknown, but there are theories of it coming from a type of wood we produced, a nice river, the wife of a pirate, or some residents just pronounced it with an accent to the British soldiers.
In mainland, people would sometimes just type the initials of the pronunciation of the character. For example, hhhhh (哈哈哈哈哈, hahahahaha), sb (傻逼 sha bi, dumbass). It can get out of hand for people who are not super familiar with their vocabularies (like me, a Hong Kong Cantonese speaker)
Alternatively, one can just use a different dialect/version of Chinese. In Hong Kong, we can have both written Chinese and spoken Cantonese mixed in the same message. We simply pick which one of them gives a shorter version of the word. For example, 回家看看有無撞其餘活動 ([Written] Go home and see [Spoken] if it conflicts with other events). Sometimes we even mix in ancient Chinese.
It was actually Synfig Studio! I draw using Inkscape and Synfig can be used to animate vector graphics specificly.