How do I add a field on the Users profile? For example, country, age etc

You need to use the show_user_profile, edit_user_profile, personal_options_update, and edit_user_profile_update hooks.

You can use the following code for adding additional fields in User section

Code for adding extra fields in Edit User Section:

add_action( 'show_user_profile', 'extra_user_profile_fields' );
add_action( 'edit_user_profile', 'extra_user_profile_fields' );

function extra_user_profile_fields( $user ) { ?>
    <h3><?php _e("Extra profile information", "blank"); ?></h3>

    <table class="form-table">
    <tr>
        <th><label for="address"><?php _e("Address"); ?></label></th>
        <td>
            <input type="text" name="address" id="address" value="<?php echo esc_attr( get_the_author_meta( 'address', $user->ID ) ); ?>" class="regular-text" /><br />
            <span class="description"><?php _e("Please enter your address."); ?></span>
        </td>
    </tr>
    <tr>
        <th><label for="city"><?php _e("City"); ?></label></th>
        <td>
            <input type="text" name="city" id="city" value="<?php echo esc_attr( get_the_author_meta( 'city', $user->ID ) ); ?>" class="regular-text" /><br />
            <span class="description"><?php _e("Please enter your city."); ?></span>
        </td>
    </tr>
    <tr>
    <th><label for="postalcode"><?php _e("Postal Code"); ?></label></th>
        <td>
            <input type="text" name="postalcode" id="postalcode" value="<?php echo esc_attr( get_the_author_meta( 'postalcode', $user->ID ) ); ?>" class="regular-text" /><br />
            <span class="description"><?php _e("Please enter your postal code."); ?></span>
        </td>
    </tr>
    </table>
<?php }

Code for saving extra fields details in database:

add_action( 'personal_options_update', 'save_extra_user_profile_fields' );
add_action( 'edit_user_profile_update', 'save_extra_user_profile_fields' );

function save_extra_user_profile_fields( $user_id ) {
    if ( empty( $_POST['_wpnonce'] ) || ! wp_verify_nonce( $_POST['_wpnonce'], 'update-user_' . $user_id ) ) {
        return;
    }
    
    if ( !current_user_can( 'edit_user', $user_id ) ) { 
        return false; 
    }
    update_user_meta( $user_id, 'address', $_POST['address'] );
    update_user_meta( $user_id, 'city', $_POST['city'] );
    update_user_meta( $user_id, 'postalcode', $_POST['postalcode'] );
}

There are also several blog posts available on the subject that might be helpful:

• Adding and using custom user profile fields
• Adding Extra Fields to the WordPress User Profile