Every file is made up of zeroes and ones, what's different between the formats is how those zeroes and ones are interpreted. When you open a mp4 in a text editor what you see is the result of the text editor interpreting the data as if it were text. Since the data doesn't actually represent text, the result is meaningless garbage.

Every single file on a computer is stored as zeros and ones, the difference between opening a file in VLC or in Notepad is how the program decode the data.

I actually have a very good analogy to explain the issue of decoding data, this happened to me in a shop.

I am a Swede, I consider myself being bilingual Swedish/English, I live and work in Sweden.

After work one day I decided that I wanted some Itallian food so I walked by Eatatly in Stockholm.

As I got to the cashier to pay, I thought I heard her speak Swedish, but as she started talking to me, I only heard gibberish, I could not understand her at all, it took me 2-3 sec to realize that she was speaking English, and when that clicked, I suddenly understood everything.

It was so weird, it was like my English comprehension was just turned off and needed to restart.

Now, this is bascially what happens when you try and open a music file in Notepad, only it can't understand music at all and doesn't have the option to give up unless it hits a hard limit.

So it uses what it knows to try and decode thw file, it takes the birnary data and decodes it as a text file, and since the music data does not corespond to proper text standards it will just do it's best and give you a long document of incomprehensable characters.

There are some interesting ways to mess with files and different programs to find/do interesting stuff.

For instance, you can hide a zip file in a JPG file: https://www.howtogeek.com/119365/how-to-hide-zip-files-inside-a-picture-without-any-extra-software/

This would only really work in hiding small ammounts of data, and will not prevent detection by law enforcement.

.docx, .xlsx, .pptx and other new office documents are actually zip files, you can open the file in 7zip and examine the file that way.

This is interesting, but I haven't found a real use for it.

All any file is is just numbers. Opening a file in a program is just interpreting those numbers. To over-simplify, in a plain text file, for example, the number 32 means "space character", and the number 10 means "move down to a new line". In an audio file, the numbers are going to have meaning related to volume and frequency of sound, at points in time.