which directory can I find wordpress built-in .mo and .po files for trasnlation

Read the code at https://developer.wordpress.org/reference/functions/wp_set_lang_dir/ to see where the language files are stored. Only the language chosen in the installation is downloaded and used.
The core code that uses the language files is in wp-includes/l10n.php and wp-includes/pomo.

As for “wordpress acceptable way of creating .po and mo. files”, WordPress core uses a tool called GlotPress to organize teams that are worldwide. This can be found at https://translate.wordpress.org. Each plugin and theme in the repository has a place there also.

There is a very helpful plugin for making your language files. It is abandoned though. You can find it in SVN here:
https://wordpress.org/plugins/codestyling-localization/#developers