The advice really only applies for very specific definitions of "learn".

Let me cite a single line from the MSVC implementation of the C++ standard library (https://github.com/microsoft/STL/blob/main/stl/inc/xtree):

    using _Scary_val = _Tree_val<conditional_t<_Is_simple_alloc_v<_Alnode>, _Tree_simple_types<value_type>,
        _Tree_iter_types<value_type, typename _Alty_traits::size_type, typename _Alty_traits::difference_type,
            typename _Alty_traits::pointer, typename _Alty_traits::const_pointer, value_type&, const value_type&,
            _Nodeptr>>>;

Also, good luck understanding this class member definition. Hope you figured out _Scary_val!

  _Compressed_pair<key_compare, _Compressed_pair<_Alnode, _Scary_val>> _Mypair;

(Hint: Empty base class optimization.)