The main component of Sketchfab is the 3D viewer, which can be viewed on the Sketchfab website or can be embedded on other webpages, as seen here.
This particular model includes description information/images on the main model page, as well as annotations to highlight key features of the model. You can also view the model using augmented reality (AR) or virtual reality (VR) if you have the necessary equipment to do so.
There are multiple ways to create a 3D model. For the example above, the 3D model was created using a process called photogrammetry and software called Agisoft Metashape. The process uses 3D photographs of a real-world object (in this case, a modern reproduction of a medieval carving) to create a digital version. One photograph from this process is visible here.