Convert Powershell Connect-PnPOnline to C#

PnP PowerShell wraps a lot of stuff under the hood to make it easy for you.

To modify your code to C#, it needs to be written as mentioned below:

var siteUrl = "https://your-sitecollection-url";
var userName = "userName";
var password = "password";

using (ClientContext clientContext = new ClientContext(siteUrl))
{
    SecureString securePassword = new SecureString();
    foreach (char c in password.ToCharArray())
    {
        securePassword.AppendChar(c);
    }

    clientContext.AuthenticationMode = ClientAuthenticationMode.Default;
    clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);

    clientContext.ExecuteQuery();

    var ServerVersion = clientContext.ServerLibraryVersion.Major;

    var site = clientContext.Site;
    var web = clientContext.Site.RootWeb;

    clientContext.Load(web, w => w.ServerRelativeUrl);
    clientContext.ExecuteQuery();

    var serverRelativeUrl = clientContext.Site.RootWeb.ServerRelativeUrl;
}

To execute this code, you will need Microsoft.SharePointOnline.CSOM nuget package in your project.