New job, having some anxiety, am I in over my head?

I just started a new job as a fullstack .net developer last week, I have about 2.5 years of experience. Most of the stuff I've worked on has been .net core APIs, occasional front-end and manual sql queries. When I interviewed, I was told that they have an angular/.net core app. But few days after I started there, I found that there's another old MVC app that's been de-prioritized, and they want my help to deciding if it could be updated or completely redone.

Our team is small, just 1 principal engineer, 1 senior, and 2 devs, and me. They are all working on the main app, and I've been tasked to look at the old app and tell them whether it should be updated or remake from scratch. It's making me a little anxious because I don't have any experience in designing app this complex, and I feel like they are expecting a lot from me even though I only 2.5 years of experience. I asked to have permission to see the code base, but they only gave me access to the old app, and finally gave me the access to see the frontend code of the main app. Every standup I feel left out because they're all working on the main app, and I just say I'm poking around the code of the old app and trying to familiarize myself with it.

I've emphasized that my strength is in working with .net core APIs but they haven't give me access to the repo. I feel like they just really want me to work on the old app. If that's the case, I feel like it's a great opportunity for me to take charge of this, but also a lot of responsibility. I'm not sure where to start, they say the product manager is designing a new flow for the app, and I feel like I can't really make any meaningful decision before knowing what the new flow is. If the new flow has complicated UI, then the app should be remake to something like angular/.net core. What are some things I can prepare myself with? Start drawing database table relationship diagrams to get a better understanding what entities I'll need in the future?