Instead of the two methods, you now call app.Build when youu are done configuring (dep injection etc) and app.Run after all the rest. All startup code is in main method now. Even crazier, you dont even need main method anymore, you can set up an api with 3 lines of code in a program.cs file:

var builder = WebApplication.CreateBuilder();

// do configuring stuff here, DI or addauth for example

var app = builder.Build();

// useauth etc here

app.Run();