They’re standard sizes and the mechanism to attach the built in doors to the appliances (where relevant) is also standardised. I’ve replaced built in fridges, washing machines, dishwashers, ovens and hobs in my own homes over the years without needing to redo anything else.

What does annoy me though is that you have to pay a premium for appliances which are ‘built in’ over the equivalent model which is not built in, despite the built in version being less complicated (no door handles, for example).