Are these theme folder paths equivalent? Place theme in /vendor/?

Question

/app/design/frontend/Magento/blank/

&&

/vendor/magento/theme-frontend-blank/

Am I correct to assume /vendor/ is always the fallback from app?

If that is true, could I install my theme to:

/vendor/joshua34/theme-frontend-mytheme/

Does theme registration or another issue prevent theme installation here?

DevDocs states Magento themes can be installed under /vendor/, "when a Magento instance is deployed from the Composer repository."

What about custom themes not installed via composer?

Answer 1

You can place you theme in any folder. Only one requirement is register it in \Magento\Framework\Component\ComponentRegistrar