Q:

How does a project manage defining the requirements needed for underlying infrastructure?

The definition of underlying infrastructure in the context of the question relates to the fabric that supports the services rendered for a given collection of systems such as example cabling, power, cooling, etc. 
Currently it isn't clear if the project is responsible for defining these or simply subscribes to existing capabilities. For example, if a given system is deployed that requires a certain amount of power, does the project simply communicate this or does it need to expand on existing requirements. 
The scope of the project thus far has been to deliver the system. 

A:

Whether the project delivers this, or whether it is an agency external to the project that feeds in to the project, is entirely down to the scope definition and practises at the company- there is no law that says what a project may or may not deliver, even for identical projects or deliverables in different organisations
Look to your project brief and your scope definitions. Determine whether Solution and/or Technical architects are present in the organisation, determine the process used in the past to define and manage the infrastructure deliveries and then figure out if you need those resources in the project team and encapsulate all the findings and arrangements in the Project Initiation Document, including Roles & Responsibilities.

