How much of a video game's art is made in the engine and how much from external programs?

Hello! First of all I apologize if this question sounds stupid or if it has been asked before. I did use the searchbox but I couldn’t find the answer I was looking for.

When game developers use an engine, how much of the art needs to be “imported” and how much of it is done in the actual engine.

For example, if someone wants to create a level that takes place in a cave and the game engine is Unity, will they be able to create the walls, the rocks, the stalagmites,the bats, the dripping water etc. in Unity or will they have to create some of the cave elements in Maya, Blender etc and then import them in the engine?

Thank you!

Ideally 100% is imported from other software, unless your game is made mostly of untextured primitive cubes and spheres.
You can bake lightmaps in Unity, do all your animtions, do stuff like built-in terrains / trees, particle fx and trailrenderers.
But there is no polygon modeling feature and no painting tools, so every model and every texture is done externally.