Overloaded chaining or stacking

See output chaining first. Generally, since << is left-associative, return X <<(ostream X, values)

... the left referenced ostream is returned and the next values in the chain added to it.

Why must << and >> be non-members,

typically friends, and not members ? You need to provide a definition of the function where the first parameter passed is of type stream as the left-side operand is always of type stream, as in ...

cout << obj;

It cannot be a member function and you would typically make it a friend to allow access to the private data of the object you are wanting to output.

See https://en.cppreference.com/w/cpp/language/operators