Paid users get access to an iframe embedding link that can be copy/pasted directly from the model page. You'll find it in the embed tab below the viewer.
Regardless if a model is set to public or private, you will need to whitelist the domains that will be allowed to embed your models. You can whitelist domains by going to the model Edit page. Read more about whitelisting domains.
Along with the standard iframe fields like width and height, there are a number of embedding options related either to the 3D model or the control widget which can be specified as properties of the iframe code. Read more about those options.
Activating API access
For developers to be able to interact with the viewer API, the last line of the iframe code should be uncommented:
<!-- ShapeDiver Viewer API reflector, uncomment the following line to use it --> <script src="//viewer.shapediver.com/v2/2.3.0/sdv_reflector.min.js"></script>