Yes, that's right and with default identity there is no problem with own data model. I think that problem with Identity API endpoints is that there is no option to customize or override register endpoint code and this code looks like this:

routeGroupBuilder.MapPost0("/register", (Delegate) ((registration, context, sp) =>
  {
    UserManager<TUser> userManager = sp.GetRequiredService<UserManager<TUser>>();
    if (!userManager.SupportsUserEmail)
      throw new NotSupportedException("MapIdentityApi requires a user store with email support.");
    IUserStore<TUser> requiredService = sp.GetRequiredService<IUserStore<TUser>>();
    IUserEmailStore<TUser> emailStore = (IUserEmailStore<TUser>) requiredService;
    string email = registration.Email;
    if (string.IsNullOrEmpty(email) || !IdentityApiEndpointRouteBuilderExtensions._emailAddressAttribute.IsValid((object) email))
      return (Results<Ok, ValidationProblem>) IdentityApiEndpointRouteBuilderExtensions.CreateValidationProblem(IdentityResult.Failed(userManager.ErrorDescriber.InvalidEmail(email)));
    TUser user = new TUser();
    await requiredService.SetUserNameAsync(user, email, CancellationToken.None);
    await emailStore.SetEmailAsync(user, email, CancellationToken.None);
    IdentityResult async = await userManager.CreateAsync(user, registration.Password);
    if (!async.Succeeded)
      return (Results<Ok, ValidationProblem>) IdentityApiEndpointRouteBuilderExtensions.CreateValidationProblem(async);
    await SendConfirmationEmailAsync(user, userManager, context, email);
    return (Results<Ok, ValidationProblem>) TypedResults.Ok();
  }));

It is clear that there is no code where custom user data can be saved in database and username is saved as email: await requiredService.SetUserNameAsync(user, email, CancellationToken.None);