Save Players Creation to be opened later

Hi Unitarians!

This should be a fairly simple question, but about half an hour of intense googling has not yielded me the result.

In my game, the player can create their own projects. For simplicity's sake, lets say they're like models. I want my player to save their model to their computer's hard drive. Then, the next time they open the game, I want them to be able to open the file and continue working on it. The file will essentially be a combination of hundreds of prefabs that the user combined together in their own fashion.

How would I do this?

Thanks in advance!

(Bonus question! I have my file full of Unity prefabs, as I said above. Is there anyway I could export this file into a .fbx file or something through Unity?)