You can use builder.Configuration. In this example, the connection string is retrieved in the second line of actual code:
using System.ComponentModel.DataAnnotations;
using Microsoft.EntityFrameworkCore;
var builder = WebApplication.CreateBuilder(args);
var connectionString = builder.Configuration.GetConnectionString("TodoDb") 
                       ?? "Data Source=todos.db";
builder.Services.AddSqlite<TodoDb>(connectionString)
                .AddDatabaseDeveloperPageExceptionFilter();
The WebApplicationBuilder.Configuration property is a Microsoft.Extensions.ConfigurationManager instance that implements IConfigurationRoot and IConfiguration, so it can be used to load config settings or use extension methods like GetConnectionString
Once the application is built, configuration is accessible through the WebApplication.Configuration property. This is just a call to Services.GetRequiredService<IConfiguration>():
public IConfiguration Configuration => 
    _host.Services.GetRequiredService<IConfiguration>();