How to add database in RESTful web application?

I am a newbie in asp.net and trying to learn things. So I created a project with code from a tutorial online(https://learn.microsoft.com/en-us/aspnet/core/tutorials/first-web-api?view=aspnetcore-2.2&tabs=visual-studio) with the the following classes.

Controllers:

namespace WebApiSample.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class TodoController : ControllerBase
    {
        private readonly ToDoContext _context;

        public TodoController(ToDoContext context)
        {
            _context = context;

            if (_context.TodoItems.Count() == 0)
            {
                // Create a new TodoItem if collection is empty,
                // which means you can't delete all TodoItems.
                _context.TodoItems.Add(new ToDoItem { Name = "Item1" });
                _context.SaveChanges();
            }
        }

        // GET: api/Todo
        [HttpGet]
        public async Task<ActionResult<IEnumerable<ToDoItem>>> GetTodoItems()
        {
            return await _context.TodoItems.ToListAsync();
        }

        // GET: api/Todo/5
        [HttpGet("{id}")]
        public async Task<ActionResult<ToDoItem>> GetTodoItem(long id)
        {
            var todoItem = await _context.TodoItems.FindAsync(id);

            if (todoItem == null)
            {
                return NotFound();
            }

            return todoItem;
        }

        // POST: api/Todo
        [HttpPost]
        public async Task<ActionResult<ToDoItem>> PostTodoItem(ToDoItem todoItem)
        {
            _context.TodoItems.Add(todoItem);
            await _context.SaveChangesAsync();

            return CreatedAtAction("GetTodoItem", new { id = todoItem.Id }, todoItem);
        }

        // PUT: api/Todo/5
        [HttpPut("{id}")]
        public async Task<IActionResult> PutTodoItem(long id, ToDoItem todoItem)
        {
            if (id != todoItem.Id)
            {
                return BadRequest();
            }

            _context.Entry(todoItem).State = EntityState.Modified;
            await _context.SaveChangesAsync();

            return NoContent();
        }

        // DELETE: api/Todo/5
        [HttpDelete("{id}")]
        public async Task<ActionResult<ToDoItem>> DeleteTodoItem(long id)
        {
            var todoItem = await _context.TodoItems.FindAsync(id);
            if (todoItem == null)
            {
                return NotFound();
            }

            _context.TodoItems.Remove(todoItem);
            await _context.SaveChangesAsync();

            return todoItem;
        }
    }
}

Models:

namespace WebApiSample.Models
{
    public class ToDoItem
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

namespace WebApiSample.Models
{
    public class ToDoContext : DbContext
    {
        public ToDoContext(DbContextOptions<ToDoContext> options)
    : base(options)
        {
        }

        public DbSet<ToDoItem> TodoItems { get; set; }
    }
}

Startup.cs:

namespace WebApiSample
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<ToDoContext>(opt => opt.UseInMemoryDatabase("ToDoList"));
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseMvc();
        }
    }
}

Program.cs

namespace WebApiSample
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CreateWebHostBuilder(args).Build().Run();
        }

        public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>();
    }
}

You can see that it doesn't use any database. How can I add a database in this project so that whenever I run it I can get the list of items already added before ?

Any ideas?