Source

File: wp-includes/general-template.php

function get_site_icon_url( $size = 512, $url = '', $blog_id = 0 ) {
	$switched_blog = false;

	if ( is_multisite() && ! empty( $blog_id ) && get_current_blog_id() !== (int) $blog_id ) {
		switch_to_blog( $blog_id );
		$switched_blog = true;
	}

	$site_icon_id = get_option( 'site_icon' );

	if ( $site_icon_id ) {
		if ( $size >= 512 ) {
			$size_data = 'full';
		} else {
			$size_data = array( $size, $size );
		}
		$url = wp_get_attachment_image_url( $site_icon_id, $size_data );
	}

	if ( $switched_blog ) {
		restore_current_blog();
	}

	/**
	 * Filters the site icon URL.
	 *
	 * @since 4.4.0
	 *
	 * @param string $url     Site icon URL.
	 * @param int    $size    Size of the site icon.
	 * @param int    $blog_id ID of the blog to get the site icon for.
	 */
	return apply_filters( 'get_site_icon_url', $url, $size, $blog_id );
}