It's impossible to infinitely enlarge a photo using data that doesn't exist. If the data is not a part of the photo, it can't magically appear. However, features exist that employ guesswork to fill in missing data. If you blow up an image large enough, it's going to become pixelated. A pixel is one point in a graphic image.
Pixels are so close together that they are seen as connected by the naked eye.
To interpolate is to estimate values that lie between known values. When you scan a photo, the actual resolution will be enhanced, or interpolated, to capture up to 90,000 pixels per square inch. It shows the raw data, and the steps to reach the final result. Photoshop is an option for the rest of us. Photoshop has features that can analyze the data included in a photo, and can then use the findings to fill in the gaps in the photo.