The “theme” is a wall-village, where the village is made up of housing units which forms a wall on one side, and a housing facing a protected center on the other side. Various reusable modules forms individual buildings, which are then joined up into a wall.
I do to an extent. At least when I am playing in the style of RC. Others there would be less pieces.
RC by default in their style makes it quite easy to want placing buildings next to each other.
The way I go about modular is that I usually make most of my buildings having a foundation of slabs, then foundation tool. Each having their various 1-4 in height where the top number would be the bottom floor. This is for the buildings. From that I don’t make stairs attached to the same blueprint. I make those separate so I can have various different stairs or parts that would be stairs.
I would probably do that for defensive walls for sure, but I am in habit of not making much big places.
Tendency to build with terrain sort of helps with the idea of modular. It is how I designed stairs to go with the 1-4 heights, whether it was a stoop, porch, or general stairs. Having many variations makes it so if I did decide to place a building from my templates, I could have a different style of stairs leading up to it.
It is a general concept, and love doing it where I need to. Modular is a way to make quite a variation in buildings.