what is the nature of compgen under zsh (as opposed to bash)?

I am in the process of replicating my development environment under Ubuntu from Macos. A good deal of which consists of porting some utility zsh scripts *from one machine to the other*. One recurring hurdle has been getting compgen to work. I've researched posts like [Bash completion does not work in ZSH/Oh-My-ZSH because COMP_WORDS is not an array](https://stackoverflow.com/questions/69675174/bash-completion-does-not-work-in-zsh-oh-my-zsh-because-comp-words-is-not-an-arra) Bottom line: so far, on Linux zsh, I am stuck with "compgen command not found". I did use locate to find a compgen somewhere under bash completions, but calling it from zsh did not work. Yes, compgen works, from bash. Back on my mac, where compgen does work, I idly did a which compgen under zsh, which returned what looks like a function definition rather than a path. which compgen : (only first 10 lines copied to give you an idea)

`
compgen () {
	local opts prefix suffix job OPTARG OPTIND ret=1 
	local -a name res results jids
	local -A shortopts
	emulate -L sh
	setopt kshglob noshglob braceexpand nokshautoload
	shortopts=(a alias b builtin c command d directory e export f file g group j job k keyword u user v variable) 
	while getopts "o:A:G:C:F:P:S:W:X:abcdefgjkuv" name
	do
		case $name in
....

` Meanwhile, under **bash**, which compgen returns nothing, presumably because it is a builtin. So where does compgen under **zsh** come from? Is it computed through the completion initialization in my **~/.zshrc** ? quite possibly bashcompinit?: (note: below is from the macos ~/.zshrc, where compgen works)

`
# The following lines were added by compinstall
zstyle :compinstall filename '/Users/me/.zshrc'
autoload -Uz compinit
compinit
# End of lines added by compinstallx

autoload -Uz bashcompinit
bashcompinit

#stop completions from $PATH on source, as per https://stackoverflow.com/a/72710684 
unset '_comps[source]'

`