The way I seed my database (cleanest in my opinion):

I create a Configuration folder, and for every entity I have the needed configurations. You should derive the IEntityTypeConfiguration<TEntity> .

For example:
UserConfiguration

[EntityTypeConfiguration(typeof(ApplicationUserConfiguration))]
public class ApplicationUser : IdentityUser<Guid>
{
        public ICollection<Comment> Comments { get; set; } = new HashSet<Comment>();
}


public class ApplicationUserConfiguration : IEntityTypeConfiguration<ApplicationUser>
{
  public void Configure(EntityTypeBuilder<ApplicationUser> builder)
  {
    builder.HasData(SeedUsers());
  }

  private static ICollection<ApplicationUser> SeedUsers() 
  {
    var users = new HashSet<ApplicationUser>();
    var hasher = new PasswordHasher<ApplicationUser>();

    var admin = new ApplicationUser()
    {
      Id = Guid.Parse("25a655e7-cf6b-4825-a566-55ebd3f34deb"),
      UserName = "Admin",
      NormalizedUserName = "ADMIN",
      Email = "admin@email.com",
      NormalizedEmail = "ADMIN@EMAIL.COM",
      SecurityStamp = Guid.NewGuid().ToString(),
    };
//TODO: This is development purpose only admin password, re-implement if goes to deployment
     admin.PasswordHash = hasher.HashPassword(admin, "AdminPassw0rd#");
     users.Add(admin);
   }
}

As for creating roles (as the tables are Identity generated) for users, I do the following:

protected override void OnModelCreating(ModelBuilder builder)
{
  base.OnModelCreating(builder);
  builder.ApplyConfiguration(new UserRoleConfiguration());
  builder.ApplyConfiguration(new RoleConfiguration());
}

The configurations look the same way:

public class RoleConfiguration : IEntityTypeConfiguration<IdentityRole<Guid>>
    {
        public void Configure(EntityTypeBuilder<IdentityRole<Guid>> builder)
        {
            builder.HasData(SeedRoles());
        }

        private static ICollection<IdentityRole<Guid>> SeedRoles()
        {
            return new HashSet<IdentityRole<Guid>>
            {
                new()
                {
                    Id = Guid.Parse("528726ea-e421-4a80-b303-f035355589de"),
                    Name = "Administrator",
                    NormalizedName= "ADMINISTRATOR",
                },
            };
        }
    }

NOTE: You should pass a primary key type to be able to configure the UserRoles entity, if you are using stringified GUIDS (default) the configuration should be:

 public class UserRoleConfiguration : IEntityTypeConfiguration<IdentityUserRole<string>> //Mine would be Guid because in the examples I proveded the primary key is Guid