Some beginners would like to ask but do not dare:
What is a file format?
To start, let’s say it is only the way the data are written in your file. In principle, it should not affect the image itself.
Ok, but an image is only a 2D array (the image is a rectangle), with optionally a third dimension to store bands. A very straightforward way of saving this data in a file is simply to copy the matrix into the file in a very plain way: 1st pixel, 2nd pixel, 3rd pixel, etc up to the last pixel of the first line, then continue on the next line, and so on to the end of the image. This is what does the Envi format. When you have bands, you can even choose if you want to write all the bands values of the same pixel in a row, or write the image of the first band first, then append the second band, etc. In any case, you’ve got something that is very similar to a copy of the computer memory where the file as to be placed when it is read.