When you are looking for brief information about a program in your PATH with its associated man pages, you would use ...

which only outputs the full path of the executable that would be run, and provides no information about man pages.

whereis searches standard binary, source, and manual page directories and returns all associated paths for a given program name, including man page locations. Unlike which, it is not limited to the executable alone. This makes it the correct choice when the goal is to find brief program info alongside its documentation.

locate searches a prebuilt file index for any file matching a pattern and is not specific to programs in PATH or their man pages.

where is not a standard Linux command; it exists in some shells like csh/tcsh as a built-in but does not report man page locations.