Why do we use polymorphisom by designing a software.and what is the aplication of templates in object oriented programing?

Quote:

I cannot find the exact solution. That's why I need a proper solution.

There is no "exact solution" to a generic question like this: it depends on so many different variables. You will have to study pretty much the whole of OOPs design and formulate your own opinion.

If your course notes do not cover it - and they probably do - then start here and read some articles, follow some links, see what you can find out: Why do we use polymorphism and templates - Google Search[^]

The point of polymorphism
Imagine you have a class that describes common attributes for graphical interface objects, and that's just that and is therefore defined as an abstract class (one that cannot generate direct instances).
Given that, you create specialized classes that derive from the abstract class that have members in common, this new class defines a button. And following this principle you specialize a class to draw panels and so on. But a common thing in GUI elements is the "contains" relationship, a panel can contain a button, a button a panel... And this is possible thanks to polymorphism, at runtime it becomes possible to arbitrarily assign objects derived from the abstract class with each other.
This was a quick attempt to bring up a use case of polymorphism.

And when it comes to object orientation, the importance is in the organization of data. For example you model a structure once, and when you need that structure again, there's no point in copying the previous lines, just generate an instance of that structure. Imagining a case, templates are useful so that the same structure can adapt to different types of data at compile time, for example, we have an array that precisely uses only integers but it would be nice if we could work with float without having to copy the implementation of that array and change the data type from int to float, right? These are templates in a basic way.