On this page, https://developers.google.com/search/docs/guides/sd-policies#completeness we can read:
Specify all required properties for your rich result type. Items that are missing required properties are not eligible for rich results.
The more recommended properties that you provide, the higher quality the result is to users. (...)
But on schema.org, I'm not able to find which are the required and which are the recommended properties.
Are these required and recommended properties only a thing which is understandable for the google search algorithms to work well, or are they intrinsic values of any properties of the schema.org vocabulary? In the latter case, how and where can I find them (I'm not able to figure it out on schema.org)?