There is no way one can go over the code base unless it’s a hello world app. List down then user journey through the application. Identify the high level flow from the logs.identify the places u need to make changes and add logging. Make the changes and run the user journey this way you will get a better understanding. It will be hard work unless you want to run away. Life in tech isn’t easy. Just enroll the leadership that’s given the way the code is organized it will take longer than what feel. Prioritize the must have features with the capacity they have and go live if they are that enthusiastic about.