Missing Startup.cs in .NET 6, how to configure?

In new .NET 6, Startup.cs is merged with Program.cs so now you can use all the configuration in Program.cs something like this to add JSON Settings (After installing Nuget package 'Microsoft.AspNetCore.Mvc.NewtonsoftJson' for it)

using Newtonsoft.Json.Serialization;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllersWithViews();
builder.Services.AddControllers().AddNewtonsoftJson(options =>
{
    // Use the default property (Pascal) casing
    options.SerializerSettings.ContractResolver = new DefaultContractResolver();
});

Also, if you want to add database connection string you can have something like this

builder.Services.AddDbContext<DBContextHere>

instead of old way "services.AddDbContext"

To Add Services in .NET 6 Program.cs

var builder = WebApplication.CreateBuilder(args);

// Add the memory cache services.
builder.Services.AddMemoryCache();

// Add a custom scoped service.
builder.Services.AddScoped<ITodoRepository, TodoRepository>();
var app = builder.Build();

To Add Custom dependency injection (DI) container in .NET 6

var builder = WebApplication.CreateBuilder(args);

builder.Host.UseServiceProviderFactory(new AutofacServiceProviderFactory());

// Register services directly with Autofac here. Don't
// call builder.Populate(), that happens in AutofacServiceProviderFactory.
builder.Host.ConfigureContainer<ContainerBuilder>(builder => builder.RegisterModule(new MyApplicationModule()));

var app = builder.Build();

Here is the complete migration guide

https://docs.microsoft.com/en-us/aspnet/core/migration/50-to-60?view=aspnetcore-5.0&tabs=visual-studio#where-do-i-put-state-that-was-stored-as-fields-in-my-program-or-startup-class

With sample and comparison of code from old to new .net version (6)

https://docs.microsoft.com/en-us/aspnet/core/migration/50-to-60-samples?view=aspnetcore-5.0

Hope it helps.