Reading large code

Hi everyone! I started an internship three months ago, and I am starting a new project where I will be continuing to develop some software started by a previous engineer.

The software is a C# WPF that follows MVVM design pattern (I had never used either C#, WPF, or MVVM but I have a C++ background so that is helpful) and its is REALLY big, so …

I wanted to know any techniques/tips for reading and understanding code (e.g. making diagrams) that may be useful, because the code uses a lot of interfaces, methods, etc.

Thanks!