before learning Pixi.js. I usually use the raw canvas API to draw 2D graphics. This is interesting, but a bit complicated. For example, drawing a star shape requires the use of BeizierCurve API, but the positions of the shape's points often are difficult to find, Therefore it is best to use third-party libraries to simplify path operations.
there are some exercises on Pixi.js homepage, which make us learn Pixi.js api. I finished an example called 'Choo Choo Train' and I think that those APIs and their construction is similar to Three.js. you'll be able to master those APIs faster if you have ever learned Three.js