question about component integration or assembly

Dear All,

I am still new to this component concept.
At this stage, I have a question about the concept and the
implementation of the components when they are combined or integrated
or assemblied with other components.

If we are a developer of a client application and wants to use
component X and Y.
The definition of component allows components be connected to each
other. I could understand how my client application would be connected
to component X and Y. However how could we connect X to Y, if don't
have the source code of X and Y?

Thanks in advance.
Han Liong
.