Very few questions are stupid, these are not.

Yes, you can definitely build an entire fps game using WebGL for rendering. Typically using JavaScript to handle physics, collision, gameplay, etc.

My current WebGL project is rendering high definition terrain, high-poly animated models, thousands of particles, shaders, sound and more over 150 frames-per-second on a 10 year old PC with a RTX 3060. I have found hardware acceleration is often not enabled in the browser, or Windows will default to using the integrated-graphics card when running the browser and that must be changed in the Windows Graphics Settings.

WebGL is a graphics API for talking right to the graphics card, supported by The Browser. ThreeJS and BabylonJS are libraries that make it easier to render 2D and 3D graphics, both use WebGL and/or WebGPU behind the scenes for rendering.

Development with HTML/CSS/JavaScript and WebGL is my favorite stack to work with. Development is fast, re-loading is quick, errors and debugging is handled directly in the browsers which have great debug information and performance tracking. No compile time and support on lots of devices.

Yes, OpenGL came first. WebGL is a JavaScript binding of a subset of OpenGL functionalities.