Where can I find the SQL to get the most used information by wordpress database?

I tried to google “wordpress sql to get post url” and I was pretty
astonished I couldn’t find any resource sharing the SQL code to join
the wp_post table with the table containing the permalink.

That is because there is no table containing the permalink. Your “challenge” and your “astonishment” are because you’ve imagined the system to work in a way that it doesn’t. The permalink isn’t stored anywhere. It is calculated based on the permalink settings in the backend. If you want to see how that happens, take a look at the source code for get_permalink(). The only thing saved to the database is the permalink pattern, which is saved to the options table under the permalink_structure key and looks something like:

/%year%/%monthnum%/%day%/%postname%/

But, of course, that varies for the numerous different structures possible.