What’s concretely the designer’s mental model?

In the book "The UX Book: Process and Guidelines for Ensuring a Quality User Experience", the authors mention in chapter 8.2 designer's mental model (also called conceptual model). They first define it as

DMM is a vision of how a system works as held by the designer.

and then

DMM is the designer's conceptualization of the envisioned system.

They say that the designer's mental model can be seen from three perspectives: ecological, interaction and emotional. However, IMHO, their definitions are a bit vague. They also do not give examples in the context of designing UI for web and mobile applications, which happens to be my case.

Suppose I am the designer of a UI for a mobile application whose purpose is to allow people to exchange, share and give away goods or items they no longer need. Given that I am the designer, then, I suppose, the designer's mental model is my mental model of how my mobile application works or should work.

But how would you define the designer's mental model if you were the designer of this specific application (i.e. what's your mental model about this application)? What would be your ecological, interaction and emotional perspectives of this application?

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Note: I am looking for a concrete answer to this specific example. I am not looking for other vague and general answers.