Applications dont really need to be well architected until they are hitting scale. Then the parts of their system that need to relieve pressure will need to be re-architected. This is almost like a case study and there are a lot of good talks on youtube from places like dropbox and facebook that explain the problem and solution. Example: https://www.youtube.com/watch?v=PE4gwstWhmc

If you dont want to do youtube case studies there are also books to read about distributed systems. Also reading about cloud architecture can help.