Another thing that is helpful, especially if you don't even have knowledge of the problem domain of the codebase: Write a glossary. As you read the code and en

Another thing that is helpful, especially if you don't even have knowledge of the problem domain of the codebase: Write a glossary. As you read the code and encounter terms/words you don't know, write them down. Try to explain what they mean and how they relate to other terms. Make it a hyperlinked document (markdown #links plus headings on github works pretty well), that way you can constantly refresh your memory of previous items while writing Items in the glossary can range from class names / function names to datatype names to common prefixes to parts of the file names (what is `core`? what belongs there?) Bonus: parts of the end result can be contributed back to the project as documentation.