Root folder questions around Upload API when using App Token Auth

The api documentation here https://developer.box.com/reference#upload-a-file specifies to use "0" for root folder when uploading a file. This suggests that we could specify a different folder if we wanted to. When using app token auth:

• how can we specify a different parent folder?
• Does the folder have to be created first manually? or, does the api create it on demand?
• Does two different apps referring to parent folder "0" upload files to same location? or, are there internal folder that gets created for different apps?
• Is there any limitation to specifying a different folder other than the root folder?