Hi there,
the original developers certainly used their own world editor to build these scenarios. With the Remastered Collection I'm making use of the backwards-compatibility of the game engine which means I can port scenarios, cars and missions from the older games with moderate adjustments where neccessary.
To provide a little more insight, there are a number of hardcoded material types that make up the scenarios (roads, buildings, grass, water), and we're lucky that these properties remained largely unchanged for the last couple of games.