Find WordPress subsite URL, supports Mercator and Domain Mapping plugin

site-url.php

<?php
$sites = array_map( 'get_object_vars', get_sites( array( 'deleted' => 0 ) ) ); //WP 4.6+

foreach ( $sites as $site ) {
	switch ( $site['blog_id'] ) {
		// If you are using Mercator (https://github.com/humanmade/Mercator) and domain mapping is active 
		case class_exists( '\Mercator\Mapping' ) && $mappings = \Mercator\Mapping::get_by_site( $site['blog_id'] ) :
			foreach ( $mappings as $mapping ) {
				if ( $mapping->is_active() ) {
					$site_url = $mapping->get_domain();
				}
			}
			break;
		// If you are using WordPress MU Domain Mapping (https://wordpress.org/plugins/wordpress-mu-domain-mapping/)
		case function_exists( 'domain_mapping_siteurl' ) && switch_to_blog( $site['blog_id']  ) :
			$site_url = domain_mapping_siteurl(false);
			restore_current_blog();
			break;
		// No domain mapping
		default:
			$network_blog_details = get_blog_details( $site['blog_id'] );
			$site_url = $network_blog_details->home;
			break;
	}
	echo $site_url;
}

Note: get_sites(), by default, only returns 100 sites. If you want all sites, use get_sites( [ 'number' => 0 ] ). You can also paginate through the sites list using get_sites( [ 'offset'' => $start, 'number' => $num ] )