The author here. You have various concepts but overall we store the start time and that's it :D the rest are details, clip, tween, easing function, anchor have surviving resizing, stagger to generate the same anim with variations. Or variations of clips, loop. Proper anim of colors in oklab, oklch, linear srgb, hsl, ... Anyway I cannot explain much it's really simple code with a userfriendly front-end.

Not the author but it‘s implemented in only two files which can be studied on Github.