Apple's AddressBook (also known as Contacts) is used in various MacOS and iOS versions as well as the iCloud system (which can sync with these). It is how contacts are stored for use in various Apple apps including phone, email, and texts.
The contact info is stored as vCards in the metadata folder. The pictures are stored as images. Everything is organized via a database, which is apparently in some variation of SQLite given the use of file extensions ending in -wal and -shm, something that database system uses for write-ahead logging.