How does Drupal store/use/access .po files?

I am trying to trouble-shoot why some of a site is showing translated strings and other parts of it are not.

Therefore I would really appreciate someone describing what happens when you import a .po file to your site.

Where does it get stored? drupal_files table?

How does it get used? Dissected and sent to locales_source table as a BLOB? If so where can you see each BLOB contents?

What are your suggestions for trouble-shooting strings that aren't showing as translated?

ie site is switched to 2nd language, go to user settings to view the emails that are sent out upon registration. Most of the core words and phrases on the page are translated but definitely not the messages themselves, despite being in the .po file

T.I.A.