This is how we modernized a legacy insurance mobile app.
We started with an incredibly open ended blue sky app design. Similar to some of the other products I've worked on, this vision was more or less a final view of what the UX team thought the app should be.
My job was to build the foundational design system and work with the engineering teams to get us to that vision.
We started with an app built in the mid-2000's.
All of the information you might need was there but to do anything useful you'd have to dig two, three or four layers deep.