For a UI app, I think it's critical to first know how the app works from a user point of view. Then pick a (small) feature, and try figure out how it's rendered to screen by: - Read high level documentation (hopefully it exists) on the overall architecture, not deep-dives in the beginning - Find text that is rendered and tracing to where it comes from - Use the debugger; set breakpoints on client + server code to see the flow of data, particularly into lower layers that get data from DBs/APIs - Hopefully it's a TypeScript project and you can use an IDE to find references and otherwise navigate from symbols to see where functions/class/fields are used