WebRequest is forbidden on Sharepoint Online - asp.net

Question

I need to create web request to specific page in SharePoint Online using Access Token in order to get certain Header information, but I keep getting 403 and I'm not sure why. After toying with access requests some more, I managed to only get 403 using clientContext, and 401 using Postman.
I can get response from grah api in my app and with postman using access token, but the problem is I need to do a web request to get real web page headers like SpRequestDuration and SPIISLatency.
I've followed steps to create my Azure AD and application.
I request user login to get authentication code for my application
loginScopes = [
'User.Read.All',
'Directory.Read.All',
'Group.Read.All',
'Sites.Read.All',
'Reports.Read.All'
'offline_access',
'https://www.sharepoint.com/AllSites.FullControl'
];
const encodedScopes = encodeURIComponent(loginScopes.join(' '));
const encodedRedirectUri = encodeURIComponent(redirectUri);
let url = `https://login.microsoftonline.com/organizations/oauth2/v2.0/authorize?
response_type=code&
client_id=${clientId}
&redirect_uri=${encodedRedirectUri}
&scope=${encodedScopes}
&prompt=select_account`;
After this returns authentication code I create ConfidentialClientApplication with authority : 'https://login.microsoftonline.com/organizations' and with my application clientId, secret, redirect etc. With newly created ConfidentialClientApplication I acquire token silently with scope '{tenantUrl}/AllSites.FullControl'and another token with other scopes.
Now with access token I successfully create ClientContext, which retrieves data from sharepoint site and I can easily make a graph API request and everything will be fine. But the problem is with creating a WebRequest to a specific site (example. {tenantUrl}/SitePages/Forms/ByAuthor.aspx)
var manager = new AuthenticationManager();
var clientContext = manager.GetAzureADAccessTokenAuthenticatedContext(tenantUrl, accessToken);
clientContext.Load(clientContext.Site);
clientContext.Load(oWebsite.Lists);
clientContext.ExecuteQuery(); // Works fine
var request = clientContext.WebRequestExecutorFactory.CreateWebRequestExecutor(clientContext,{tenantUrl}/SitePages/Forms/ByAuthor.aspx).WebRequest;
request.Method = "GET";
//request.Headers.Add("Authorization", $"Bearer {accessToken}");
clientContext.ExecutingWebRequest += delegate (object sender, WebRequestEventArgs e)
{
e.WebRequestExecutor.WebRequest.Headers.Add("Authorization", "Bearer " + accessToken);
};
await request.GetResponseAsync(); // Throws 403

Answer 1

with sharepoint online
string siteUrl = ConfigurationManager.AppSettings["siteURL"];
SecureString passWord = new SecureString();
string pass = ConfigurationManager.AppSettings["userPassword"];
string user = ConfigurationManager.AppSettings["userName"];
foreach (char c in pass.ToCharArray()) passWord.AppendChar(c);
ClientContext clientContext = new ClientContext(siteUrl);
clientContext.Credentials = new SharePointOnlineCredentials(user, passWord);
return clientContext;
without
clientContext.ExecutingWebRequest