I was looking at stuff like /bin/bash, and I wanted to make my own file like that that's full of aliases, but I need to know the file extension. Please tell me.
You shouldn't add stuff in /bin as this directory is handled by the OS. Use a custom directory in your homedir or /opt/monkeyman/bin for example, and use an extension only if you find it useful for some reason. Unix/Linux doesn't require or even make use of a file extension for commands, it only wants an executable to have the executable permission set.