How are bitmap images stored on a computer system?

Bitmap images, also called raster images -- are a collection of colored squares/dots on a grid each chunk of this grid and each one is called a pixel -- and the amount of pixels is the resolution of the image. Each pixel may have one of 1,658,135 different color values (255 channels red, 255 green, 255 blue). In the bitmap file format (.bmp) each pixel has its information saved individually with no chunking, shortcuts or other methods applied to it to reduce its filesize. Modern raster formats (e.g. JPG, GIF, PNG) are essentially bitmap images that have compression applied to it in order to reduce filesize and sometimes implement extra features like transparency (also known as alpha). The concept of image compression is a beyond the scope of this question.