Yes, absolutely free. You can use them for an any purpose as you wish.

The best way is to click to RMB (Right Mouse Button) and use "Save Link As" (MS Windows users only) option in menu.

The process of creation 3D models take some time, that's why not all of them are available.

In our opinion it is a 2k, because of good enough level of details could be achieved and minimum disk space could be used.

This is a limitation of development/hosting platform.

This is a limitation of the platform. Maximum file size limited to 25 Mb. that's why 4k textures (normal map and SSS) could be slightly color-corrected and blured.