Shell built-ins are only accessible to the shell, so external commands can not run them. e.g. the external
time command can only run other external commands. External commands can not run shell built-ins, shell aliases, or shell functions.
Note that shells are themselves external commands (e.g.
/bin/bash etc), so external commands can run the shell with
-c, e.g. compare
sh -c 'time echo foo' to
sh -c '/usr/bin/time echo foo', or execute a shell script which uses built-ins.
If there happen to be external commands with the same name as a shell built-in (e.g.
kill, ...) then external commands can run those, but even if they perform the same basic task they are likely to be slightly different (e.g. different command line options, different output format, different capabilities - built-in
kill can kill by shell job number, external
Shell built-ins (e.g.
command) can execute either built-in or external commands, as well as shell aliases and functions.