A program (binary, or executable) is a file on disk somewhere, in a recognized format. Common formats include ELF on Linux, ...

Using stdin as a file object allows Python to read input, which is piped to it from other commands, and writing to stdout allows it to continue passing the information through the piping system.

The bash man page will tell you a little about the eval command, though you would have to scroll down nearly 5,000 lines to find it. There are, after all, no man pages for built-ins.