Hi Harold,
there are tools called "Unity Ripper". I think, I can tell you this, because it is somewhat public knowledge.
With this tools you can do exactly what you have asked for.
And as you already said, you will lose some informations, so you have to edit those ripped stuff in Unity.
For complex scenes, it is sometimes almost as much work as doing the scene yourself, but at least you will have the models.
BUT most likely this is against some copyright rules and some creators wouldn't be happy about this.
Please: at least NO SHARING (without asking the creator)!
Creating an Unity asset is a form of piracy protection. It is meant to publish stuff that might contain copyrighted content.
So, tools like mentioned above, might even be illegal to use for some content!
Please take this into consideration.