I'm no expert but currently currently getting into some coding after a bit of a break. Usually there is documentation, but you never know how relevant it still is. For this example with chromium, it looks like each folder has a readme.md, one even links to a dev wiki/guide and in there you can find google docs with diagram of the architecture :) For other projects there might be focus on doxygen which sort of collects the comments from the sourcefiles and puts it into for example html witch class trees etc.