Open 0xr.space, press Open and choose the .sldprt. It is read in the browser from the display mesh SOLIDWORKS saves inside every part, the triangles it last drew on screen, so the part looks as it did to its designer, at its real size in millimetres. Parts from old SOLIDWORKS versions and from 2015 on both open, and so does an assembly (.sldasm) saved before 2015.
The solid itself is stored as Parasolid, which only Siemens' licensed kernel can read, so a part saved without display data has nothing to draw and is refused with a message. An assembly saved from 2015 on keeps each part apart and does not open yet; open its parts one by one, or export the assembly as STEP or GLB. Colours and appearances are not read: the part is shown in one grey, and curved faces are as smooth as SOLIDWORKS drew them.
No. The file is read in the browser, with no plug-in, licence or account.
It was saved without display data, so only the Parasolid solid is in it. Export it from SOLIDWORKS as STEP or STL and open that instead.
One saved before 2015 opens whole. A newer one does not yet: open each part, or export the assembly as STEP or GLB.
No. It is read on the device, and nothing is sent to a server unless you create a share link yourself.