Creating stars that move along a specific path

Questions
2

I dont think the documentation lacks explanation. Everything beyond the documentation does involve your imagination, because each project or idea has its very own way of implementation. This involves experience in the language/framework. Threejs provides tons of examples to learn from other peoples experience. You can have wood, screws and tools to build a home, but no one can ever explain you, how you imagine it to look/feel/function. Also your first house will not be as good as your third.

There are many ways to detect the bottom of the page. One example:

window.onscroll = function(ev) {
    if ((window.innerHeight + window.scrollY) >= document.body.offsetHeight) {
        // Reached bottom
    }
};

You can either store animations directly in your 3D model when using Blender for example. Here you would animate your objects and export them, then in ThreeJs you just start and stop the animation.

Or you can animate your model by manipulating the 3D objects directly in Three. Here you would import your model and manipulate for example the position.

For your usecase its probably best to do the animations in Threejs directly.
Here is a example to animate sth along a path.
And heres another discussion about sth similar

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