What is the difference/purpose between "require" and "package-install"?

Question

I am trying to understand the subtle nuances of Emacs' packaging system(s). I do not find that anyone has really clarified or explained the difference between two, what I assume to be, key components of Emacs package system: require and package-install.

I commonly see code like the following in many init.el files (illustrative example):

(require 'package) ; Use Emacs package system
(add-to-list 'package-archives '("melpa-stable" . "https://stable.melpa.org/packages/") t) ; Add MELPA package repository as a place to go lookup for packages
(package-initialize) ; Turns Emacs package system on
; ... refresh code omitted for brevity
(unless (package-installed-p 'helm) ; Only installed "helm" if we don't see it as already installed
    (package-install 'helm)
)

(require 'helm) ; Get helm to actually use
; ...

The thought process for understanding what is going on:

• require seems to be called for package but the "package" for package does not need to be "installed". Is this internal somehow?
• Is package needed for use-package to work but not require, correct?
• For helm (in this example) we must do/use package-install (with some checks) to first "install" helm and then require to actually use it. Is this correct?
• If we did (use-package helm ... ) then we would not need either require or package-install 'helm, right?

To extrapolate:

If one wanted to use any particular package, they first need to make sure they have an equivalent package-install call / statement prior to invoking require (or just utilize a use-package which will "merge" both steps) on that particular package, correct?

I just want to be sure I am properly setting up my usage of packages and so forth. Emacs is very confusing when it comes to packages, installing, requiring, etc. ... at least that I find.

---

Q/A Housekeeping Notes:

According to common advice, there are important differences in the Emacs major versions, so I would like an explanation for each (Emacs version 25, 26, and 27, respectively) for others who may come across the same question but on an older version (if it matters).

I am currently on Emacs 27.2 on macOS Catalina (just to be clear).

In an attempt to preempt "duplicate of ..." or "searching will find ...": In my searching, I found require vs. package-initialize? but it does not look like anyone really answered the question to satisfaction (or clearly).

Answer 1

Think of a package as a bunch of e-lisp and maybe other files. Many of these (such as the package package itself) come as part of your emacs (these are said to be built-in) and so need no further installation. Others, such as pdf-tools or helm are located in repositories on the interwebs such as melpa and so must be downloaded onto your computer before they are available to emacs. This is (part of) what package-install does.

A separate concern is to load a package into your running emacs so that you can use its functionality. This is what require does. It can only load stuff that is on your system. This is why there is a package-install that may have to be done first.

Finally use-package can do both things in one shot:

(use-package foo)

is essentially a fancy wrapper around (require 'foo) while

(use-package foo
  :ensure t)

calls package-install on foo, if necessary.

For a given package, you can do describe-package (bound to C-h P in vanilla emacs) to see where it comes from and whether it is installed.

There is more to say here: once a package is installed, the magic of autoloads means that require may be quite unnecessary: see the previously referenced post require vs. package-initialize? for this.