Displaying third party software licenses



so far I understood that we should display a licenses section in our commercial app, with the list of open source libraries and plugins we use. (most of them are under the MIT license or the Apache 2.0 license).

However, could anyone here explain me how it works when the library you are using is not open source? Should the plugin contain a License file with the description of their terms? Should the company decide what we display (e.g. Copyright XX 2019)? Or is it not needed to display it at all because I am paying for it?

I’m not sure if this is the place to ask, but if not, I would appreciate if someone could redirect me to the right place.

Thanks in advance.