Great achievement to have read so much code. Honestly, I felt a little depressing for not having read this much code. As an aside, how do you read codebases?

Great achievement to have read so much code. Honestly, I felt a little depressing for not having read this much code. As an aside, how do you read codebases? Do you read every single line to understand what's going on? Or you get general idea about design/architecture? What are the proven strategies to read code bases?