Fallout 4 uses modular kits to create its levels.
Joel Burgess and Nathan Purkeypile worked together using modular techniques.
They explain what it takes to create kits like these.
Bethesda has been using modular kits for level design since a long time.
This goes back to Daggerfall which already used them.
Later games continued this trend.
In Morrowind everything was snapped to a grid.
This however made the player feel the presence of the kits.
As they were always going into cardinal directions.
When creating a kit there is a really important concept:
This refers to the grid, size and shape of the pieces
Staying inside the footprint is important.
Going over it means overlapping with other kit pieces.
Another important topic is:
Having a proper pivot can really help Level Designers.
When creating a kit one may thing that hero pieces are the most important.
( Hero pieces refer to story specific assets or assets that are important or mark landmarks )
But, in reality...
... the most important is what players will see 90% of the time.
One advantage of working with kits → Styles
If all kits follow same logic, you can change one for another.
Then you suddenly have visual level variety for free.
The way that they solved doors and frames was with the concept of:
If the hole shape/size matches, you can pug whatever plug you want in that hole.
To handle destruction they opted to have multiple variants.
That way you can have multiple destruction pieces in the same room and they won't feel the same.
At the same time, destruction can lead to a completely different kit to create transitions.
Finally, one of the best ways to hide monotony:
By using them, even if the wall is a repeating texture, it can feel completely custom made.