Why does ASP.NET Core define both WebApplicationBuilder and WebApplication?

It seems like WebApplicationBuilder is a factory, which can build multiple WebApplications. But why would a web API application ever have more than a single WebApplication?

All configuration, like DI, configuration, and logging, belongs to the WebApplicationBuilder, while middleware belongs to the WebApplication?

Bonus: Does the WebApplication include a complete Kestrel web server for serving the application referred to at the host?

Example:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApi(builder.Configuration.GetSection("AzureAd"))
    .EnableTokenAcquisitionToCallDownstreamApi()
    .AddMicrosoftGraph(builder.Configuration.GetSection("DownstreamApi"))
    .AddInMemoryTokenCaches();

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

References:

https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.builder.webapplication?view=aspnetcore-8.0

https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.builder.webapplicationbuilder?view=aspnetcore-8.0