At the request of a comment on a different video, I made a tutorial on how to create a compound planetary gearbox in FreeCAD.

First I cover the design considerations in Excel to determine the right number of teeth and tooth sizing for a compound planetary gear, and then I hop into FreeCAD and use the FCGears Workbench to add all the gears.

Note: As pointed out by JasonRobards2 in the comments, the ring gears the way I make them in this video aren't valid internal gear profiles. The correct way to make the herringbone rings would be to use the Involute Gear from the Part Design menu with the External Gear property set to False and then do a pair of subtractive helixes from a cylinder to form the rings.