I did my dorm room once using the constellations as they appeared at the moment I was born. You can find free websites that will plot the sky as it appeared at any time and place. I used little glow in the dark star stickers, and a black light to charge them. I even had a little dot on a string attached to a motor that made a satellite fly across the sky.

Many file formats have what is literally called a "Magic Number" at the very beginning of the file. If the program sees that magic number, they know it is that file format.

https://en.wikipedia.org/wiki/List_of_file_signatures