Best Of

Probably easiest to use fill surface and a couple of mirror features.

This is an old post, but for clarification for future users:
To actually move that document to the new folder, you will need to use another undocumented endpoint:

POST : /api/globaltreenodes/folder/{:fid}

BODY:

{  

  "itemsToMove" : 
    [ 
      { 
        "resourceType" : "document",    
        "id" : <docid>    
      }  
    ]
}

David,

Going through that lesson myself, and I noticed that some step explinations are missing. So one has to do a bit of figuring out as to what steps are not being mentioned. Just a heads up for you.

https://learn.onshape.com/courses/fundamentals-multi-part-part-studios

Indeed, in Onshape each "body" is a part, and don't worry too much about the "planes"…
Think of a multi-part studio more like an "in context" design assembly in SolidWorks where the parts are drawn in their "correct" location relative to the origin.
Also you will soon find out (if you haven't already…) that the individual part planes and origin are NOT included in the assembly environment so they become less relevant.

Thanks for the info.

One thing that I can mention, is that most features that create geoetry give the option if you want to create a new body or add to an existing body: