As a Free user
If you are on a free Basic plan, you can make your models downloadable in order to download your model files. You can always make them not downloadable again later.
As a Pro user
Pro (and above) users can download all their own models without setting them to be free downloadable to the rest of the community. This works both on-site for all formats (original, glTF, and USDZ), and through the Download API for our glTF and USDZ transcoded versions.
Don't worry, your models are not available to anyone else unless you allow it as usual through download settings.
Only published models may be downloaded.
As a Store seller
Sketchfab users accepted to sell their 3D models in the Sketchfab Store can download their own models if they set them as available for sale.
Sellers will find the same Download your model section under the model page Properties.