Built-in cabinets are usually built on site except for unfinished cabinet bodies. Laminates are glued on site, adjustments are made on-site. Backs of cabinets and other hidden sections remain unfinished to save costs. The finished cabinets look snug against walls and ceilings.
In the old days, actually not too long ago, the contractor or so called designer was the one you looked whenever you wanted to renovate your home. Finding a plumber, an electrician, a mason, a tiler, a carpenter was difficult. Not everyone wanted to work with each other.