Remastering old films to high-definition (HD) or 4K quality is possible because of the inherent high resolution of the original film stock used to capture the footage. Even though the screen resolutions of the time did not support HD or 4K, the film itself contained much more visual information than what was displayed on the screens of that era.
The process of remastering involves carefully scanning the original film negative at a high resolution, often 2K or 4K, and then using digital restoration techniques to clean up any damage, grain, or other artifacts. This high-resolution digital copy can then be used to create new masters for Blu-ray, 4K, or other modern formats, allowing the film to be presented with vastly improved visual quality compared to previous home video releases that were limited by the constraints of standard definition.