THIS link suggests that /app directory contains all configuration, templates (.html.twig files) and translations, and the /src directory contains PHP code.
However, HERE we can see that .css and .js files are loaded from /src/AppBundle/Resources/public directory and not /app/Resources/public directory. What is the best practice? How should I professionaly approach this problem, and where should I keep all the templates, .js and .css files?
I like to put css and javascript that serve for general purposes in /app/Resources/public and the ones specific to the concerned bundle in /src/YourBundleNameBundle/Resources/public. Like that when I want to see a code specific for a bundle I will just go to that bundle , when it's a general purpose code I would be sure that it's in the root. I have seen expert people doing it and it makes sense for me.
It's a matter of choice and there is no right and wrong answer.