Try to find an opensource project that really interests you and go through the whole architecture. Try to think of new features, implement them and when you thi

Try to find an opensource project that really interests you and go through the whole architecture. Try to think of new features, implement them and when you think they might be of interest to others, create a pull request. When you think you understood how everything works, continue with another project. Over time you will recognize that the same patterns occur in different projects and even languages. For the beginning I'd suggest to stay away from frameworks that do too much magic. If you are interested in web, start with Flask, not Spring.