This code snippet returns the collection of all the URLs of web sites from the Site Collection including top level site and its sub-sites using CSOM PnP Core Component.

PnP.GetAllWeburls.cs

//Author: Shantha Kumar T
//Assembly Reference Used: OfficeDevPnP.Core, Version=2.4.1605.0, Culture=neutral, PublicKeyToken=3751622786b357c2
//Supports: SharePoint Online, SharePoint 2013+

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;
using OfficeDevPnP.Core;

namespace SampleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            string siteUrl = "https://sharepointonline.sharepoint.com";           

            AuthenticationManager authManager = new AuthenticationManager();
            //Interactive Login to SharePoint site - Opens a Online signin page to authenticate the user
            var context = authManager.GetWebLoginClientContext(siteUrl);

            IEnumerable<string> webUrls= context.Site.GetAllWebUrls();            
            string weburl = "";
            foreach(var url in webUrls)
            {
                weburl += url + "\r\n";
            }
            Console.WriteLine(weburl);

            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
        } 
    }
}