Everything is stored as 0 1 in digital , binary format file or other format file
So when we are trying to open a Executive file with hex editor it could show all the random ASCII character that will be produced based on 7, 0 1 bits that randomly created that ( anything ) file because it is 0 1 at the storage, in memory.
So why it shows strange character that are not ASCII char?
A hex reader doesn't just parse 7 bit per 7 bit or 8 bit? it read some meta data in file and then read based on that?
Your hex editor is choosing to decode the bytes not as ASCII but as some other character encoding.
You are right the ASCII character set has 128 codepoints and the ASCII character encoding encodes them in single bytes in the range 0 to 127. Since bytes in an arbitrary file could range from 0 to 255 and the ASCII character set isn't used much for text files, decoding as ASCII wouldn't reveal as much information about potential text as a more likely character encoding and would reveal information about only half of the values of binary files.
A hex editor's job is to display and allow you to edit bytes. Additional presentations and editing capability are extra features. Many do present text, some allow search and replace of text. Some even work with other data formats such as decimal integers, multibyte integers, floating point, etc.
There is no text but encoded text. So, to support text, a hex editor—and any other program (including compilers)—must choose or allow you to choose a character encoding. For byte values that can't be decoded using that encoding, a dot or question mark is often substituted in the text display of a hex editor.
If you don't find which character encoding is used by your hex editor, you could test it by creating a file with byte values 0 to 255, see what it displays and match it against the many, many possibilities. It might be one that your operating system uses for your "default". In Windows cmd, go chcp; In Linux terminal, go locale.