> I remember being amazed by the sophistication of the animations some Flash developers could put together using it and I always wondered if the tooling made it easier, or if somebody painstakingly drew cells one by one and then spliced them together with an otherwise relatively dumb video creator.

Flash/Flash Pro/Adobe Animate (it has been renamed now) is sophisticated animation software that just happened to have the flash plugin as one output format. A large number of cartoons for broadcast tv have been made in it (https://en.wikipedia.org/wiki/List_of_Flash_animated_televis...) and it still exists despite the demise of the flash plugin. It's definitely not just the same as splicing individual frames together.