Once an optical image has been digitized and saved as a file, picture detail is limited by the density of pixels. If you zoom and, at the same time, increase the pixel count to avoid a blocky appearance, the software will try to guess what the added pixels should be in terms of colour and brightness.
No software can tell with 100% accuracy what goes in between adjacent pixels, because it has no way of knowing what the original analog picture was. It can only make an intelligent guess by interpolation. Even a simple software like Irfanview can produce quite good results.
When using Irfanview to resize an image, use the resample mode instead of a simple resize.