How can ‘admin_email’ be set?

Where is the value for admin_email set?

$email = get_option('admin_email');

How can I change it?

In WordPress backend go to: Settings > General and the field ‘Email Address’.

Or in php: update_option(‘admin_email’, ‘your_email@abc.com’);
https://codex.wordpress.org/Function_Reference/update_option