The id_rsa.pub file is basically some encrypted text in the .ssh folder under the home folder in Linux and is used for public-key cryptography. It uses the .pub file format. But why does this file format also happen to be the file extension for Microsoft Publisher? Microsoft Publisher isn't involved with this kind of encryption in any way, so why does id_rsa.pub also use .pub? Also, how do computer program distinguish between these uses?
Nobody enforces file extension choices.
The file extension .pub was selected for a public key because it is short for public.
The file extension .pub was selected for a Microsoft Publisher document because it is short for Publisher.
That they conflict is unfortunate.