ZSH: .zprofile, .zshrc, .zlogin - What goes where?
This is an attempt to write a canonical QA for this issue, as per the Meta post:  Where is the list of canonical questions stored for Ask Different? I expect it to be periodically edited with the goal of becoming a comprehensive information resource.
What should be used in ZSH on a Mac
I posted a more narrowly scoped question on Unix &amp; Linux and got some clarification on how these files &quot;work.&quot;  Here's the summary of that answer and what I've learned in my research as to what, in my opinion should be used in a ZSH environment on a Mac.

.zprofile
.zlogin and .zprofile are basically the same thing - they set the environment for login shells; they just get loaded at different times (see below).  .zprofile is based on the Bash's .bash_profile while .zlogin is a derivative of CSH's .login.  Since Bash was the default shell for everything up to Mojave, stick with .zprofile.

.zshrc
This sets the environment for interactive shells.  This gets loaded after .zprofile.  It's typically a place where you &quot;set it and forget it&quot; type of parameters like $PATH, $PROMPT, aliases, and functions you would like to have in both login and interactive shells.

.zshenv (Optional)
This is read first and read every time.  This is where you set environment variables.  I say this is optional because is geared more toward advanced users where having your $PATH, $PAGER, or $EDITOR variables may be important for things like scripts that get called by launchd.  Those run under a non-interactive shell so anything in .zprofile or .zshrc won't get loaded.  Personally, I don't use this one because I set the PATH variable in my script itself to ensure portability.

.zlogout (Optional)
But very useful!  This is read when you log out of a session and is very good for cleaning things up when you leave (like resetting the Terminal Window Title)


For an excellent, in-depth explanation of what these files do, see What should/shouldn't go in .zshenv, .zshrc, .zlogin, .zprofile, on Unix/Linux.

Some Caveats
Apple does things a little differently so it's best to be aware of it.  Specifically,  Terminal initially opens both a login and interactive shell even though you don't authenticate (enter login credentials).  However, any subsequent shells that are opened are only interactive.
You can test this out by putting an alias or setting a variable in .zprofile, then opening Terminal and seeing if that variable/alias exists.  Then open another shell (type zsh); that variable won't be accessible anymore.
SSH sessions are login and interactive so they'll behave just like your initial Terminal session and read both .zprofile and .zshrc
Order of Operations
This is the order in which these files get read.  Keep in mind that it reads first from the system-wide file (i.e. /etc/zshenv) then from the file in your home directory (`~/.zshenv) as it goes through the order.
.zshenv → .zprofile → .zshrc → .zlogin → .zlogout