All shells have a number of built-in commands which are executed in the shell's own process.
When you enter a command, the shell checks to see if it is a built-in command, and if so it executes it. If it is not a buit-in the shell forks a new process in which to execute the command. Some shells have more built-in commands than others. Check the manual page for your shell for details. The Copyright for the UNIXhelp deliverables rests with the Computing Services of the University of Edinburgh.