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Cue in Concerned Client #382716: "but then how do we update the website independently"

NetlifyCMS :-)

Needs server side is, most Concerned Clients have a cpanel shitsite with PHP and mysql

caching is not about loading a static version of your website, you could cache parts of the page and/or regenerate whole pages if content changes; diverging from the intent of the post you could also talk about caching php objects in some persistent way etc.. none of which is covered by out of vanilla wp even if the core exposes an object caching api https://codex.wordpress.org/Class_Reference/WP_Object_Cache.

This means "even if your WordPress site is as simple as you can imagine, there's still PHP running to make it happen"