In general, it's up to the individual modeller.

Google's statement of terms to users about uploads says, "You or a third party licensor, as appropriate, retain all patent, trademark and copyright to any Content you submit, Post or display on or through the Service ..."

So full ownership is with the individual.

Google's statement of terms to users about downloads says, "The Service includes both content owned or controlled by Google as well as content owned or controlled by third parties and licensed to Google (collectively, the "Materials"). <p>"Google authorizes you to view and download a single copy of the Materials solely for your personal, non- commercial use. You may not sell or modify the Materials or reproduce, display, publicly perform, distribute, or otherwise use the Materials in any way for any public or commercial purpose without the written permission of Google." But that clause about permission from Google applies to the collective Materials, not an individual item, which is clearly coverted in the first quote.

I would start with the presumption that models are for non-commercial, personal use only, with the usual kinds of "Fair Use" as would apply to any other copyrighted work.

Beyond that, some modellers would be happy to share, depending on the venture. Some would be happy with public credit, some would want a free copy of whatever it is. Some would be willing to sell outright, some to license. Terms will vary, it's all up to the individual.