Access SharePoint REST API from .net core using Azure AD registered app with certificate

Program.cs

using Microsoft.Identity.Client;
using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Security.Cryptography.X509Certificates;
using System.Threading.Tasks;

namespace SharePoint.RestApi
{
    class Program
    {
        const string ClientId = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";
        const string CertificateFileName = "example.sharepoint.pfx";
        const string Authority = "https://login.windows.net/example.com";
        static string[] scopes = new string[] { "https://example.sharepoint.com/.default" };

        static async Task Main(string[] args)
        {
            var app = ConfidentialClientApplicationBuilder.Create(ClientId)
                .WithCertificate(GetCertificate())
                .WithAuthority(Authority)
                .Build();
            var aquireTokenResult = await app.AcquireTokenForClient(scopes).ExecuteAsync();
            using (var httpClient = new HttpClient())
            {
                httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", aquireTokenResult.AccessToken);
                httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Accept", "application/json;odata=verbose");
                var requestMessage = new HttpRequestMessage(HttpMethod.Post, "https://example.sharepoint.com/_api/web?$select=Title");
                var responseMessage = await httpClient.SendAsync(requestMessage);
                var responseContent = await responseMessage.Content.ReadAsStringAsync();
                Console.WriteLine(responseMessage.StatusCode);
                Console.WriteLine(responseContent);
            }
        }

        static X509Certificate2 GetCertificate()
        {
            var certificatePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, CertificateFileName);
            return new X509Certificate2(certificatePath);
        }
    }
}

Read the article Granting access via Azure AD App-Only to learn how to set up an Azure AD app.

Also the article Get Azure AD app-only access token using certificate on .NET Core is very interesting, it provides you some background.