WebGL != three.js

WebGL is a custom language used for graphics card programming;

WebGL - Wikipedia
WebGL (Short for Web Graphics Library) is a JavaScript API for rendering interactive 2D and 3D graphics within any compatible web browser without the use of plug-ins.

three.js library make this more developer friendly from JavaScript; far from "easy"

Fundamentals - three.js manual

"Three.js is often confused with WebGL since more often than not, but not always, three.js uses WebGL to draw 3D. WebGL is a very low-level system that only draws points, lines, and triangles. To do anything useful with WebGL generally requires quite a bit of code and that is where three.js comes in. It handles stuff like scenes, lights, shadows, materials, textures, 3d math, all things that you'd have to write yourself if you were to use WebGL directly."

three.js editor

Hello vertex attributes - Web APIs | MDN

<script type="x-shader/x-vertex" id="vertex-shader">
  #version 100
  precision highp float;

  attribute float position;

  void main() {
    gl_Position = vec4(position, 0.0, 0.0, 1.0);
    gl_PointSize = 64.0;
  }
</script>

<script type="x-shader/x-fragment" id="fragment-shader">
  #version 100
  precision mediump float;
  void main() {
    gl_FragColor = vec4(0.18, 0.54, 0.34, 1.0);
  }
</script>

demo

 Draw With WebGL Flowers @codepen.io