question

Hi @AmitRawat-3636 ,
Per my test, We can get file author by CSOM. Please refer to following code

 List targetList = clientcontext.Web.Lists.GetByTitle("Documents");
 ListItem oItem = targetList.GetItemById(2);
 CamlQuery oQuery = new CamlQuery();
 oQuery.ViewXml = @"<View><Query><Where>
 <Eq>
 <FieldRef Name='LinkFilename' />
 <Value Type='Text'>Document.docx</Value>
 </Eq>
 </Where></Query></View>";
     
 ListItemCollection oItems = targetList.GetItems(oQuery);
 clientcontext.Load(oItems);
 clientcontext.ExecuteQuery();
     
 oItem = oItems.FirstOrDefault(); 
 File targetFile = oItem.File; 
 clientcontext.Load(targetFile); 
 User oUser = targetFile.Author; 
 clientcontext.Load(oUser);
 clientcontext.ExecuteQuery();
     
 Console.WriteLine("Author: " + oUser.LoginName);

If we need list owner. We need to use SPList.Author property returns the SPUser field.

         using(SPSite site = newSPSite(strURL))  
         {  
             using(SPWeb web = site.OpenWeb())  
             {  
                 SPList list = web.Lists.TryGetList(listName);  
                 if (list != null) {  
                     SPUser listAuthor = list.Author;  
                     string authorName = listAuthor.Name;  
                     Console.WriteLine("List Author Name: {0}", authorName);  
                     retry = false;  
                 } else  
                 {  
                     Console.WriteLine("List with the name \"{0}\" not exists Please try some other name.", listName);  
                     retry = true;  
                 }  
             }  
         }  

---------update---------

We can get author.Title but it seems there is no UserPrincipalName property in SharePoint 2019. Here is the sample for reference

      Web web = clientContext.Web;
      List list = web.Lists.GetByTitle("TestList");
      clientContext.Load(list, l => l.SchemaXml);
      clientContext.ExecuteQuery();
      string schemaxml = list.SchemaXml;
      XmlDocument xmlDoc = new XmlDocument();
      xmlDoc.LoadXml(schemaxml);
      XmlNode listElement = xmlDoc.GetElementsByTagName("List")[0];
      string authorid = listElement.Attributes["Author"].Value;
      User author = web.GetUserById(Convert.ToInt32(authorid));
      clientContext.Load(author);
      clientContext.ExecuteQuery();
      Console.WriteLine(author.Title);
      Console.ReadLine();

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         Web web = clientContext.Web;
         List list = web.Lists.GetByTitle("TestList");
         clientContext.Load(list, l => l.SchemaXml);
         clientContext.ExecuteQuery();
         string schemaxml = list.SchemaXml;
         XmlDocument xmlDoc = new XmlDocument();
         xmlDoc.LoadXml(schemaxml);
         XmlNode listElement = xmlDoc.GetElementsByTagName("List")[0];
         string authorid = listElement.Attributes["Author"].Value;
         User author = web.GetUserById(Convert.ToInt32(authorid));
         clientContext.Load(author);
         clientContext.ExecuteQuery();

         Console.WriteLine(author.Title);
         Console.ReadLine();