What will happen when a video's resolution is not as high as that of the screen (of Apple's products)

By default, no. It is though very common for video to be "upscaled" meaning basically that it is resized to fill more of the screen. This doesn't improve it's quality - some times it can actually degrade quality (when the source and target resolutions are not scaled by an integer factor).

However in general you cannot say either yes or no to your question. A program or app can choose to do its own quality improvement of videos before displaying them. Usually such quality improvements comes with a set of drawbacks, or at least that it only actually works for a subset of videos.