We can connect SQL Server in ASP.NET Core applications in multiple ways.

Option 1:

Use entity framework to connect SQL Server in ASP.NET Core applications

Refer to this tutorial: 

Build ASP.NET Core Web API with CRUD Operations Using Entity Framework Core

Option 2: 

.Net 6 Simplifies a lot of tasks and introduces WebApplicationBuilder which in turn gives you access to the new Configuration builder and Service Collection

var builder = WebApplication.CreateBuilder(args);

Properties

• Configuration : A collection of configuration providers for the application to compose. This is useful for adding new configuration sources and providers.

• Environment : Provides information about the web hosting environment an application is running.

• Host : An IHostBuilder for configuring host-specific properties, but not building. To build after configuration, call Build().

• Logging: A collection of logging providers for the application to compose. This is useful for adding new logging providers.

• Services : A collection of services for the application to compose. This is useful for adding user-provided or framework-provided services.

• WebHost : An IWebHostBuilder for configuring server-specific properties, but not building. To build after configuration, call Build().

To add a DbContext to the Di Container and configure it, there are many options however the most straightforward is

builder.Services.AddDbContext<SomeDbContext>(options =>
{
   options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection"));
});

Required Nugets packages:

• Microsoft.EntityFrameworkCore
• Microsoft.EntityFrameworkCore.SqlServer to use UseSqlServer

Reference: WebApplicationBuilder Class  Thanks to Microsoft.