Lots of simple shell functionality is provided as builtins. See the bash manual for a list of builtins inherited from the Bourne Shell[1] and that are bash-specific[2].

Note that some of these are part of the POSIX standard, so there's not a huge amount of variance allowed between shells which are being compliant. Obviously, PowerShell doesn't adhere to this.

[1] https://www.gnu.org/software/bash/manual/html_node/Bourne-Sh...

[2]: https://www.gnu.org/software/bash/manual/html_node/Bash-Buil...

Here's a couple of pages listing the builtins in bash: https://www.gnu.org/software/bash/manual/html_node/Shell-Bui...

And zsh: http://zsh.sourceforge.net/Doc/Release/Shell-Builtin-Command...

Powershell doesn't really have "builtins" per-se, every cmdlet is executed in the process of the shell itself but they're all contained within loadable modules (though 2-3 modules are loaded with the shell startup by default), Invoke-WebRequest is contained within Microsoft.PowerShell.Utility for example.