There are lots of ways.

First, everything is sketched on paper after they act it out and or shoot it for reference or rotoscoping. Unless rotoscoped, usually with fundamental shapes, forms, "skeletons", without clothing, etc., they'd shoot rough pencil tests. They critically look at their rough pass to see how the rough pencils can be cleaned up, possibly in stages and "tweened" by an inbetweener, an animators assistant who fills in the stuff between the important "key" frames, often with notes about motion to the side. Much of this "rough" art and film has value and is saved for future reference. The cleaned up and tweened pages are then traced on clear plastic by inkers then the outlines are filled in by the paint department and ready for the final camera.

All of these stages are version records to be referenced and traced as required.

But here's the most important part... Drawing is nice, but it's a fundamental understanding of timing, physics, and movement that the good animators have. All of the sketches are a process of refining and defining the timing and movements and performances and this is where the real hard work comes in. Now if you have the physics, weight shifting, timing, movements, and performances already established, all you then need is a mediocre animator who can trace and draw well to do a decent "reinterpretation" of what's been established.

All of that makes sense to me, but feel free to ask if you want me to elaborate on anything.

Also, 30+ years ago no one considered the technology of today where we can side-by-side everything to "catch them cheating" or shortcuts.