AppSettings.cs

using Microsoft.Extensions.Configuration;

public class AppSettings
{
    static private readonly IConfiguration _config = new ConfigurationBuilder()
        .SetBasePath(Directory.GetCurrentDirectory())
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        .AddYamlFile("appsettings.yaml", optional: false, reloadOnChange: true)
        .Build();

    public static string? ConnectionString(string name = "DefaultConnection") => _config.GetConnectionString(name);

    public static string? GetSection(string path) => _config.GetSection(path).Value;

    public static T? GetValue<T>(string path) => _config.GetValue<T>(path);

    public static string? GetValue(string path) => _config.GetValue<string>(path);
}

AppSettings.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <RootNamespace>MailSender</RootNamespace>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <PublishSingleFile>true</PublishSingleFile>
    <SelfContained>true</SelfContained>
    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="6.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="6.0.1" />
    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="6.0.0" />
    <PackageReference Include="NetEscapades.Configuration.Yaml" Version="3.1.0" />
  </ItemGroup>

  <ItemGroup>
    <None Include="appsettings.yaml" CopyToOutputDirectory="Always" />
  </ItemGroup>

</Project>