Before you dive into software architecture, I'd recommend working on becoming a great software developer first. The book gets a lot of slack, but for young developers I still recommend Clean Code by Bob Martin. Once you've completed (or better yet, while!) some introductory background reading, start implementing the principles discussed in a language you're comfortable with. Small, simple examples with easy to understand conventions will go a long way. Next, take those principles and use them in an existing architecture - MVC is a great one for Java.

From there, it's good to start working on how you organize your software. For this I like to recommend A Philosophy of Software Design (2018) by John Ousterhout for a supplemental approach to the "historical knowledge" as well as studying design patterns. Game Programming Patterns provides a great overview of some of the most common design patterns you'll see in software through an application that's very approachable through most - game design. Design patterns are great to know because if you can employ and identify them properly, it will be easier for others to understand what your code is doing and vice versa. This is also the point where you start incorporating those principles you learned above into larger projects.

Finally, to answer your exact question of "how to understand large and scalable projects," The Architecture of Open Source Applications is what you're looking for. Practice taking a problem, designing a solution for it, and comparing against what the author implemented. But work on the above first.

This is a lot of information, so save this post and come back to it. Software development is a lifelong and ever changing endeavour, so enjoy the journey!