In what part of the WordPress core does the users table and usermeta table get joined?

I’m not 100% what you are asking, it seems like several questions? But here goes:

 $meta_value = get_user_meta($user_id, $key, $single);

For example:

 $first_name = get_user_meta($user_id, 'first_name', true);

As for adding hooks I think this answer might be what you are looking for?

UPDATE

Based on some follow up comments I’ll add:

  1. The function get_user_metavalues($user_ids) from /wp-includes/user.php will return an array of user data arrays.

  2. The function get_userdata() retrieves user values using get_user_metavalues($user_ids) from /wp-includes/pluggable.php and returns a user data object.

  3. User Meta is managed through the generic metadata functions found in /wp-includes/meta.php so if you are looking for a SQL JOIN between wp_users and wp_usermeta you are not likely to find one. That file includes these functions:

add_metadata($meta_type,$object_id,$meta_key,$meta_value,$unique=false)
update_metadata($meta_type,$object_id,$meta_key,$meta_value,$prev_value="")
delete_metadata($meta_type,$object_id,$meta_key,$meta_value="",$delete_all=false)
get_metadata($meta_type,$object_id,$meta_key='',$single=false)
update_meta_cache($meta_type,$object_ids)

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