The changing requirements part is mystifying part of Agile. How do you build the a proper foundation without knowing the base requirements it needs to support d

The changing requirements part is mystifying part of Agile. How do you build the a proper foundation without knowing the base requirements it needs to support day one and plan for day 30/60/90/365 days needs. Its almost like its written by the PO/PM without really understanding anything else but their slice of work and how they can make it easier for them. Money and time (for us humans) are finite, big things at lease have to be targeted precise efforts, a good plan and architecture that maps back and supports the requirements.