public class Class1 {
     public int Id { get; set; }
     public NestedProp NestedProp { get; set; }
}

public class NestedProp {
     public decimal Prop1 { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Class1>(e =>
            {
                e.OwnsOne(t => t.NestedProp);
                e.Property(p => p.NestedProp.Prop1).HasColumnType("NUMERIC(38, 16)");
            });
}