ok now it works

NuGet/NuGetClient.cs

@@ -0,0 +1,73 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+using NuGet.Converters;
+using NuGet.Models;
+using NuGet.Versioning;
+
+namespace NuGet;
+
+public class NuGetClient
+{
+    private readonly HttpClient _client;
+    private readonly Uri _packageBaseAddress;
+    private readonly Uri _registration;
+
+    public static JsonSerializerOptions SerializerOptions { get; }  = new(JsonSerializerDefaults.Web)
+    {
+        Converters =
+        {
+            new VersionJsonConverter(),
+            new VersionRangeJsonConverter(),
+            new FrameworkJsonConverter(FrameworkNameFormat.ShortFolderName),
+        },
+        WriteIndented = true
+    };
+
+    private NuGetClient(HttpClient client, Uri packageBaseAddress, Uri registration)
+    {
+        _client = client;
+        _packageBaseAddress = packageBaseAddress;
+        _registration = registration;
+    }
+
+    public Task<Stream> GetPackageContentStreamAsync(string id, NuGetVersion version)
+    {
+        return _client.GetStreamAsync(new Uri(_packageBaseAddress,
+            new Uri($"{id.ToLower()}/{version}.nupkg", UriKind.Relative)));
+    }
+
+    public Task<Registration> GetPackageRegistrationAsync(string id, NuGetVersion version)
+    {
+        return _client.GetFromJsonAsync<Registration>(
+            new Uri(_registration,
+                new Uri($"{id.ToLower()}/{version}.json", UriKind.Relative)),
+            SerializerOptions
+        )!;
+    }
+
+    public Task<RegistrationRoot> GetPackageRegistrationRootAsync(string id)
+    {
+        return _client.GetFromJsonAsync<RegistrationRoot>(
+            new Uri(_registration,
+                new Uri($"{id.ToLower()}/index.json", UriKind.Relative)),
+            SerializerOptions
+        )!;
+    }
+
+    public Task<CatalogEntry> GetPackageCatalogEntryAsync(string url)
+    {
+        return _client.GetFromJsonAsync<CatalogEntry>(url, SerializerOptions)!;
+    }
+
+    public static async Task<NuGetClient> CreateFromIndexUrlAsync(string indexUrl)
+    {
+        var client = new HttpClient();
+
+        var index = await client.GetFromJsonAsync<NuGetIndex>(indexUrl, SerializerOptions);
+        
+        var (packageBaseAddress, _, _) = index!.Resources.First(b => b.Type.Id == "PackageBaseAddress");
+        var (registration, _, _) = index!.Resources.First(b => b.Type.Id == "RegistrationsBaseUrl");
+        
+        return new NuGetClient(client, new Uri(packageBaseAddress), new Uri(registration));
+    }
+}