If you never tasted chocolate - how would you know what chocolate tastes like.
In the days of film, different camera formats (film sizes) had different focal length lenses that produced a standard field of view.
35mm film cameras used a 50mm lens; medium format cameras used an 80mm lens; 4x5 view cameras use a 135mm lens, and so on. The image produced from each of these cameras/lenses was essentially the same, but the information recorded on a 35mm negative was considerably less than the 4x5 negative, thereby limiting the extent of enlargement when printing. Thus it is with image sensor sizes, and to some extent, number of megapixels.
However, to say that a 70mm lens changes to a 105mm lens is wrong. The focal length and characteristics of the lens don't change, only the effective field of view by the smaller sensor. Cropping or magnification factor of a camera is meant to be a simple guide if you have experience with 35mm film cameras. It means little to a first-time digital camera user. What you see in the viewfinder, or viewing screen, is what you get, regardless of what focal length lens is attached.