Modular sets don't necessarily need to be based on "squares" and "tiles" One thing that might help you answer your own question is to take your reference or concept image and draw over it with colored lines to highlight objects you perceive as modular or could be made modular. Look for things that appear they fit together…
That's way too broad a topic to get meaningful answers in a thread, in my opinion. Start by reading the environment section of the wiki http://wiki.polycount.com/wiki/Environment For more specific questions about modularity which seems to be something you're after there's a lot of good stuff here…