An aside, I don't use Processing very often but every time I've dipped into it I've found it simple and enjoyable.
Processing is a flexible software sketchbook and a language for learning how to code. Since 2001, Processing has promoted software literacy within the visual arts and visual literacy within technology. There are tens of thousands of students, artists, designers, researchers, and hobbyists who use Processing for learning and prototyping.
Why do you need a language to learn to code when you can just... learn to code in one of the more simple languages then switch later to something more complex such as Java? Python's pretty simple to get started with.
Processing is Java + sophisticated graphics and animation capabilties: you create a canvas and you can then draw to it. It has a bunch of functions and libraries for use by artists and graphics people. It’s solid and performant. And there is a Python mode these days if you like significant whitespace.
p5.js is Processing ported to JavaScript.
Python tends to really suck for stuff like that as a beginner, because you immediately butt heads with the terrible package management/library situation and why does the pip installer crash with an unhelpful error message and what the hell is a virtualenv vs conda vs pipenv and oh also why does every one of those install scripts crash and ohmygod I just want to draw a line and I see advice like https://www.google.com/url?sa=t&source=web&rct=j&opi=8997844... screw this I'm out.
The Arduino thing came later, the language and tools (IDE, etc) was quite mature by then.