What is never cached

Whatever the settings say:

• Signed-in visitors. No setting changes this: one stored copy of a page is served to everybody, so a page built for somebody signed in would show the next visitor their name and toolbar.
• Form submissions — a POST is never a cache hit.
• The admin, the login screen, the REST API and cron.
• Pages that set a cookie, or that refuse caching with Cache-Control: no-store, no-cache or private, or an Expires date already in the past.
• Password-protected posts, previews and the customiser.
• With WooCommerce: the cart, the checkout, account pages, and every page for anyone with something in their cart.
• With Easy Digital Downloads: the checkout and the receipt pages.

Anything that says "not cached" in Diagnostics will name the exact rule — including the ones only checked while the page is being built, such as the cart and the checkout.