What programming language are you interested in? Usually you can go to GitHub, find a project with a lot of stars for the language you want and start browsing

What programming language are you interested in? Usually you can go to GitHub, find a project with a lot of stars for the language you want and start browsing the code. It is worth paying attention to the general attributes of good codebase: tests, documentation, build automation. If all this is there, then most likely you will definitely find something interesting or new for yourself.