An environment is not a game. Models are not a game. You can create an environment and models all you want, but assuming you get them into a viewer, the models just sit there doing nothing.
Content itself is not published that way. There are several asset stores where people will buy content, but unless that content happens to be a good match for whatever game someone is making, the content sees no sales.
Dropping them into Unity is easy enough: place your model files inside your Unity project. Drop them into a scene, position them where you want them, and you've got something that views your models. That still isn't a game, but it can be enough for someone to passively view your environment. It certainly is not enough to get through Steam's processes.
To turn it into a game, you need a design for your game that is actually a game. You need code to create gameplay experiences, you need audio to go with your experiences, and animations to make things move. You need UI elements and other visual effects. You need people to test the game and shake out all the bugs.
To get them in Unity you can drop your Maya files into Unity and it will automatically extract an FBX format and use them; when you change the file it will automatically extract and update. Or, if you want more control, you can export to many format options such as fbx, obj, or collada files, and Unity can use them directly.
