We make a racing game but how to find common language with the track?

Over the past year, we have been creating the physics of the game, inventing the gameplay and player behavior, creating cool cars and effects for them, and now the momentous moment has come, we switched from the cube track as the beginning of work on its final appearance, and stopped ... our tracks should be similar to the tracks from Motorstorm (as will be seen in the video) but we just don't know where to start, how is this done for such racing games? Is it better to create a terain and then put 3D models of the environment on it? Or do it all together, how do you create in your head a complete vision of the final track, maybe it's better to create everything from scratch in 3D max? Share your experience as we are stuck and don't know what to do next ... in the video you can see the track, how to do something similar in Unity?

https://www.youtube.com/watch?v=abmeUrM979c

Depends on the levels. I prefer to create the terrain randomly in tools like World Creator 2, export the height map into Unity, add vegetation and roads. Then re-iterate. And once you're happy fine-tune it. Because the ideas will keep on coming while you play your own game.

Use creative tools which allow you to modify it without too much work afterwards.

Then polish it.

Here's an example of how you can get running very fast in Unity:

https://www.youtube.com/watch?v=5hkACkvf-AU

In the above video it'd be just different biomes you use, e.g. for desert. The problem in the Motorstorm video you posted is that you'd need meshes for the vertical rocks. Nothing unsolvable, but I'd use some mesh generator for that instead of placing the vertical rocks manually. Either way, for the first prototype you wouldn't need it anyway. You may as well ask @raoul from Easy Roads if he could create meshes like that. After all Easy Roads added roads along mountains and that might fit your situation. I'd also use Easy Roads for the inital roads themselves.