Objects can respond to the GDL global for the frame number, and the fly through will show the objects moving as long as the option is ticked to rebuild for every frame in the fly through dialog.
See the GDL Reference Guide in the Documentation folder of your AC installation in the Misc section - Global Variables > Fly-through information (page 292 of the English AC 18 GDL guide): GLOB_FRAME_NR in particular, but there are others. Every object that you want animated must be hand-modified. A lot of work.
10 to 15 years ago, lots of people were doing this, but I haven't seen any recent discussion. Technique has not changed. but I think most people are animating objects in Artlantis, C4D and other external packages now as it is far less complex than modifying objects and you have better control on an animation timeline than trying to compute what an object should be doing at a specific frame.
One of the forum moderators • AC 26 USA and earlier • macOS Ventura 13.2, iMac Pro 10-Core, Radeon Pro Vega 64