var сlient = new System.Net.WebClient();
var html = webClient.DownloadString("https://любойсайт.com/");
using System.IO;
using System.Net.Http;
var httpClient = new HttpClient();
await using var response = await httpClient.GetStreamAsync("https://example.com");
await using var file = File.OpenWrite("site.html");
await response.CopyToAsync(file);