Canon has the EF mount
Nikon has the Nikon mount
Sony has its own mount
Olympus etc has the fourthirds mount
Sigma, Tamron, Tokina and a few other brands make lenses that are compatible with Canon and Nikon cameras. Some work better than others, some don't at all. e.g. old Sigmas cannot work with newer Canon cams even though they fit into the mount properly - the electronics are no longer compatible
But you cannot take a Nikon-compatible Tamron and fit it into a Canon camera and vice versa.
Hope you are clear.