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

SMSS.exe is an executable file that stands for Session Manager Subsystem. It is a crucial component of the Microsoft Windows operating system and starts running when you turn on your computer.

Being a builtin doesn’t necessarily mean that a command doesn’t exist as an executable file in the file system. Some builtins are only builtins. They’re part of the shell. Others are also files.