Looking for various case studies of network architecture and migration with concurrent availability

I am working as an intern/apprentice in a 6 month project, that is migrating an existing enterprise network that has been messed with by multiple vendors, and needless to say, runs inefficiently. I will have the guidance of my mentors while participating in the project, but it was suggested to familiarize myself in the high level overview of what this kind of project entails, and find some case studies of similar projects so I start getting an idea of what we will all be tackling.

The company has about 400 employees, spread between three campuses, the first of which is the headquarters, the second is a manufacturing plant in the same city, and the third is another business they recently purchased in another city. There is a fourth network at play that is the datacenter accessed via two separate ISPs, and located in another state.

We will build out the new network, load test, test transition, migrate, and have them running concurrently for a month before taking down the old network.

I'm new to the industry, so I am looking for some resources or shared documentation created by architects and network admins that give some details of how to plan a project like this, how it was executed, and the lessons learned. Kind of the sort case study you'd learn about in a college course, so I know going into the experience what are the high level issues I need to look at.